As other examples of correlation we may cite the correlation which seems to obtain between short canine teeth and the absence of a hairy covering to the body. This phenomenon is observed both in men and pigs. Hairless dogs almost invariably have their teeth but poorly developed.
Darwin called attention to the connection between a short beak and small feet in pigeons; we see the same phenomenon in the dwarf breed of ducks known as call-ducks.
A curious correlation exists between fowls’ eggs with brown shells and the incubating habit. Fanciers have long tried in vain to produce a hen that lays brown eggs without becoming “broody” at certain seasons.
Among fowls, long legs are invariably correlated with a short tail, as is well seen in the Malay breed. This correlation may explain the short tails of wading birds. Short-legged fowls, like Japanese bantams, have long tails, and it is significant that the short-legged Weka Rails (Ocydromus) of New Zealand have unusually long tails for the family. In this connection we may say that the tail-like plumes of the cranes are not tail-feathers, but the tertiary feathers of the wings. As egrets also have long trains of plumes growing from the back, it cannot be said that the short tail of the vast majority of the waders is due to the fact that these birds would be at a disadvantage were their caudal feathers long.
Isolation
Isolation is a most important factor in the making of species. It is a factor to which Darwin failed to attach sufficient importance, and one which has been to a large extent neglected by Wallaceians.
Divergence of Character
We have seen how a species can be improved or changed by natural selection. All those individuals which have varied in a favourable direction have been preserved, and allowed to leave behind them offspring that inherit their peculiarities, while those which have not so varied have perished without leaving behind any descendants. Thus the nature of the species has changed. The old type has given place to a new one. Instead of species A, species B exists. This is what Romanes has called monotypic evolution—the transformation of one species into another species. But any theory of the origin of species must be able to answer the question, Why have species multiplied? How is it that species A has given rise to species B, C, and D, or, while itself continuing to exist, has thrown off sister species B and C? How is it that in the course of evolution, species have not been transmuted in linear series instead of ramifying into branches? This ramification of a species into branches has been termed by Romanes polytypic evolution. It is easy to see how natural selection can bring about monotypic evolution, but how can it have effected polytypic evolution? To use Darwin’s phraseology, how is it that divergence of character has come about? Darwin’s reply to this question is (Origin of Species, p. 136), “from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.
“We can clearly discern this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural power of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in its conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous. The more diversified in habits and structure the descendants of our carnivorous animal become, the more places they will be enabled to occupy. What applies to one animal will apply throughout all time to all animals—that is, if they vary—for otherwise natural selection can effect nothing.” Darwin was, therefore, of opinion that natural selection is able to bring about polytypic evolution. Darwin tacitly assumes, in the illustration he gives, that the various races of the carnivorous animal are in some way prevented from intercrossing; for if they interbreed indiscriminately, these races will tend to be obliterated.