Zoology : an elementary text-book / [Sir A.E. Shipley].

  • Arthur Shipley
Date:
1920
Catalogue details

Licence: Public Domain Mark

Credit: Zoology : an elementary text-book / [Sir A.E. Shipley]. Source: Wellcome Collection.

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    remain the same, this straggle for existence will only weed out those individuals least perfectly adapted to their environment, so that the species will be kept up to a high level of adaptation to its surroundings. This elimination of imperfect individuals which results in the survival of the fittest is known as Natural Selection. Thus we can well imagine that if white-haired individuals turned up amongst hares, they would be more conspicuous and hence more easily discovered by the animals which prey on hares. If however the circumstances of a species change, a different class of individuals will survive. For instance, if for the greater part of the year the country inhabited by the hares were covered by snow, as is.the case in the North of Canada, the whitest-haired individuals would have the best chance, and from generation to generation would be selected until the colour of the hare was totally changed. The progressive modification of species by the agency of natural selection is called evolution. If the modification tends towards simplification of structure it is called degeneration, if on the contrary it tends towards great complexity it is spoken of as differentiation. So far the theory shows how a species will become slowly modified as its surroundings change. But it has been postulated that distinct species have arisen from the same ancestors. It is of course not difficult to see that if a species is distributed over a wide area the conditions in different portions may vary independently of one another, and hence the species may become modified in one place in one direction and in another situation in a different direction by the agency of natural selection. So long however as the species inhabits a continuous area this tendency to split up into divergent groups will be checked by inter-breeding between the sections of the species which are thus becoming modified in different directions. But if through geographical changes the species becomes divided into groups of individuals cut off from access to another, then no inter-breeding can take place and in time two species will be formed. Thus when birds have been blown far out to sea and have colonised a distant island they have often given rise to a new species. The same result may be brought about by the sea overflowing a part of the area inhabited by the species, an event which we know from geology to have often occurred. The important fact to be borne in mind is that at bottom the evolution of several species out of one is due to the formation of colonies, and that the same causes which have led to the differences between the American and the Englishman have acted again and again in the world’s history so