Definitions

combinable

History of zoology (1859–1912)

This article considers the history of zoology in the years up to 1912, since the theory of evolution by natural selection proposed by Charles Darwin in 1859.

Charles Darwin gave new direction to morphology and physiology, by uniting them in a common biological theory: the theory of organic evolution. The result was a reconstruction of the classification of animals upon a genealogical basis, fresh investigation of the development of animals, and early attempts to determine their genetic relationships.

Early 20th century work

After publication of his work The Origin of Species, Darwin became interested in the animal and plant mechanisms that confer advantages to individual members of a species. Much important work was done by Fritz Muller (Für Darwin), by Hermann Müller (Fertilization of Plants by Insects), August Weismann, Edward B. Poulton and Abbott Thayer. There was considerable progress during this period in the field that would become known as genetics, the laws of variation and heredity (originally known as thremmatology). The progress of microscopy gave a clearer understanding of the origin of the egg-cell and sperm-cell and the process of fertilization.

Mendel and zoology

Mendel's experiments on cultivated varieties of plants were published in 1865, but attracted little notice until thirty-five years later, sixteen years after his death (see Jeptiism). Mendel tried to gain a better understanding of heredity. His main experiments were with varieties of the edible pea. He chose a variety with one marked structural feature and crossed it with another variety in which that feature was absent. For example, he hybridized a tall variety, with a dwarf variety, a yellow-seeded variety with a green-seeded variety, and a smooth-seeded variety with a wrinkle-seeded variety. In each experiment, he concentrated on one character; after obtaining a first hybrid generation, he allowed the hybrids to self-fertilize, and recorded the number of individuals in the first, second, third, and fourth generations in which the chosen character appeared.

In the first hybrid generation, nearly all the individuals had the positive character, but in subsequent generations the positive character was not present in all individuals: half had the character and half did not. Thus the random pairing of two groups of reproductive cells yielded the proportion 1 PP, 2 PN, 1 NN, where P stands for the character and N for its absence - the character was present in three-quarters of the offspring and absent from a quarter. The failure of the character to distribute itself among all of the reproductive cells of a hybrid individual, and the limitation of its distribution to half only of those cells, prevents the swamping of a new character by interbreeding. The tendency of the proportions in the offspring is to give, in a series of generations, a reversion from the hybrid form PN to a race with the positive character and a race without it. This tendency favours the persistence of a new character of large volume suddenly appearing in a stock. The observations of Mendel thus favoured the view that the variations upon which natural selection acts are not small but large and discontinuous. However, it did not appear that large variations would be favoured any more than small ones, or that the eliminating action of natural selection upon an unfavourable variation could be checked.

Much confusion arose in discussions of this topic, because of defective nomenclature. Some authors used the word mutation only for large variations that appeared suddenly and which could be inherited, and fluctuation for small variations, whether they could be transmitted or not. Other authors used fluctuation only for small, acquired variations due to changes in food, moisture and other features of the environment. This kind of variation is not heritable, but the small variations to which Darwin attached importance are. The best classification of the variations in organisms separates those which arise from congenital variations from those which arise from variations of the environment or the food-supply. The former are innate variations, the latter are "acquired variations". Both innate and acquired variations include some which are more and some which are less obvious. There are slight innate variations in every new generation of every species; their greatness or smallness so far as human perception goes is not of much significance, their importance for the origin of new species depends on whether they are valuable to the organism in the struggle for existence and reproduction. An imperceptible physiological difference might be of selective value, and it might carry with it correlated variations which may or may not appeal to the human eye but which are of no selective value themselves.

The views of Hugo de Vries and others about the importance of saltatory variation, the soundness of which was still not generally accepted in 1910, may be gathered from the article Mendelism. A due appreciation of the far-reaching results of correlated variation must, it appeared, give a new and distinct explanation of large mutations, discontinuous variation, and saltatory evolution. The analysis of the specific variations of organic form to determine the nature and limitation of a single character, and whether two variations of a structural unit can blend when one is transmitted by the male parent and the other by the female, were yet to be determined. It was not clear whether absolute blending was possible, or whether all apparent blending was only a more-or-less minutely subdivided mosaic of non-combinable characters of the parents.

Another important development of Darwin's conclusions deserves notice. The fact of variation was familiar: no two animals, even of the same brood, are alike. Jean-Baptiste Lamarck hypothesised that structural alterations acquired by a parent might be transmitted to the offspring, and as these are acquired by an animal or plant as a consequence of the action of the environment, the offspring would sometimes start with a greater fitness for those conditions than its parents started with. In turn, it would acquire a greater development of the same modification, which it would transmit to its offspring. Lamarck argued that, over several generations, a structural alteration might thus be acquired. The familiar illustration of Lamarck's hypothesis is that of the giraffe, whose long neck might, he suggested, was acquired by the efforts of a short-necked race of herbivores who stretched their necks to reach the foliage of trees in a land where grass was deficient, the effort producing a longer neck of each generation, which was then transmitted to the next. This process is known as 'direct adaptation'.

Such structural adaptations are acquired by an animal in the course of its life, but are limited in degree and rare, rather than frequent and obvious. Whether acquired characters could be transmitted to the next generation was a very different issue. Darwin excluded any assumption of the transmission of acquired characters. He pointed to the fact of congenital variation, and showed that congenital variations are arbitrary and non-significant.

Congenital variation

At the beginning of the twentieth century, the causes of congenital variation were obscure, although it was recognised that they were largely due to a mixing of the matter which constituted the fertilized germ or embryo-cell from two individuals. Darwin had shown that congenital variation was all-important. A popular illustration of the difference was this: a man born with four fingers only on his right hand might transmit this peculiarity to at least some of his children; but a man with one finger chopped off will produce children with five fingers. Darwin, influenced by some facts which seemed to favour the Lamarckian hypothesis, thought that acquired characters are sometimes transmitted, but did not consider that this mechanism was likely to be of great importance.

After Darwin's writings, there was an effort to find evidence for the transmission of acquired characters; ultimately, the Lamarckian hypothesis of transmission of acquired characters was not supported by evidence, and was dismissed. August Weismann argued from the structure of the egg-cell and sperm-cell, and from how and when they are derived in the growth of the embryo from the egg, that it was impossible that a change in parental structure could produce a representative change in the germ or sperm-cells.

The only evidence that seemed to support the Lamarckian hypothesis were the experiments of Charles Brown-Séquard, who produced epijeptilism in guinea-pigeptys by bisection of the large nerves or spinal cord, which led him to believe that, in rare instances, the artificially-produced epilepsy and mutilation of the nerves was transmitted. The record of Brown-Séquard's original experiments was unsatisfactory, and attempts reproduce them were unsuccessful. Conversely, the vast number of experiments in the cropping of the tails and ears of domestic animals, as well as of similar operations on man, had negative results. Stories of tailess kittens, puppies, and calves, born from parents one of whom had been thus injured, are abundant, but failed to stand experimental examination.

Whilst evidence of the transmission of an acquired character proved wanting, the a priori arguments in its favour were recognized as flawed, and cases which appeared to favour the Lamarckian assumption were found to be better explained by the Darwinian principle. For example, the occurrence of blind animals in caves and in the deep sea was a fact which even Darwin regarded as best-explained by the atrophy of the eye in successive generations through the absence of light and consequent disuse. However, it was suggested that this is better explained by natural selection acting on congenital fortuitous variations. Some animals are born with distorted or defective eyes. If a number of some species of fish are swept into a cavern, those with perfect eyes would follow the light and eventually escape, leaving behind those with imperfect eyes to breed in the dark place. In every succeeding generation this would be the case, and even those with weak but still seeing eyes would escape, until only a pure race of blind animals would be left in the cavern.

Transmission

It was argued that the elaborate structural adaptations of the nervous system which underlie instincts must have been slowly built up by the transmission to offspring of acquired experience. It seemed hard to understand how complicated instincts could be due to the selection of congenital variations, or be explained except by the transmission of habits acquired by the parent. However, imitation of the parent by the young account for some, and there are cases in which elaborate actions must be due to the natural selection of a fortuitously-developed habit. Such cases are the habits of 'shamming dead' and the combined posturing and colour peculiarities of certain caterpillars (Lepidoptera larvae) which cause them to resemble dead twigs or similar objects. The advantage to the caterpillar is that it escapes (say) a bird which would, were it not deceived, attack and eat it. Preceding generations of caterpillars cannot have acquired this habit of posturing by experience; either a caterpillar postures and escapes, or it does not posture and is eaten - it is not half eaten and allowed to profit by experience. Thus, we seem justified in assuming that there are many movements of stretching and posturing possible to caterpillars, that some had a fortuitous tendency to one position, some to another, and, that among all the variety of habitual movements, one is selected and perpetuated because it happened to make the caterpillar look more like a twig.

Record of the past

Man, compared with other animals, has the fewest instincts and the largest brain in proportion to body size. He builds up, from birth onwards, his own mental mechanisms, and forms more of them, and takes longer in doing so, than any other animal. The later stages of evolution from ape-like ancestors have consisted in the acquisition of a larger brain and in the education of that brain. A new feature in organic development makes its appearance when we set out the facts of man's evolutionary history. This factor is the record of the past, which grows and develops by laws other than those affecting the perishable bodies of successive generations of mankind, so that man, by the interaction of the record and his educability, is subject to laws of development unlike those by which the rest of the living world is governed.

References

Search another word or see combinableon Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature