The advantages of prolificacy are so apparent that it is unnecessary to dilate upon them. Nearly as important as excessive fertility is the ability on the part of the parents to look after their young ones.
Every successful species possesses in a special degree at least one of the above attributes. It is interesting to take in turn the various species which are most widely distributed and consider to what extent they possess these several qualities.
Let us now consider a factor in evolution which is nearly as important as natural selection itself—we allude to the phenomenon of correlation.
Correlation
We may define correlation as the interdependence of two or more characters. This phenomenon is far more common than the majority of naturalists seem to think. It very frequently happens that one particular character never appears in an organism without being accompanied by some other character which we should not expect to be in any way related to it.
Darwin called attention to this phenomenon. “In monstrosities,” he writes, on page 13 of the Origin of Species (new edition), “the correlations between quite different parts are very curious, and many interesting instances are given in Isidore Geoffroy St Hilaire’s great work on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some instances of correlation are quite whimsical: thus cats which are entirely white and have blue eyes are generally deaf; but it has been lately stated by Mr Tait that this is confined to the males.
“Colour and constitutional peculiarities go together, of which many remarkable cases could be given among animals and plants. From the facts collected by Heusinger, it appears that white sheep and pigs are injured by certain plants, whilst dark-coloured individuals escape. Professor Wyman has recently communicated to me a good illustration of this fact: on asking some farmers in Virginia how it was that all their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes), which coloured their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the ‘crackers’ (i.e. Virginia squatters) added, “we select the black members of a litter for raising, as they alone have a good chance of living.’
“Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.
“Hence, if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of the correlation of growth.”
The great importance of the principle of the correlation of organs is, that natural selection may indirectly cause the survival of unfavourable variations, or of variations which are of no utility to the organism, because they happen to be correlated with organs or structures that are useful.