An interesting case, showing that qualities which are life-preserving under certain severe conditions exist in some varieties of a species and not in others, was recorded some eight years ago. After a very severe "blizzard" 136 common sparrows were found benumbed on the ground by Professor Bumpus at Providence, United States. They were brought into a warm room and laid on the floor. After a short time seventy-two revived and sixty-four perished. They were compared to see if the survivors were distinguished by any measurable character from those which died. It was found that the survivors were smaller birds (the sexes and young birds being separately compared) than those which died, and were lighter in weight by one-twenty-fifth than the latter. Also, the birds which survived had a decidedly longer breastbone than those which died.
Similarly, the late Professor Weldon found that in the young of the common shore-crab, taken in certain parts of Plymouth harbour, those with a little peculiarity in the shape of the front of the shell survived when those without this peculiarity died. Many thousands were collected and measured in this experiment. It is not necessary to suppose that the distinguishing mark of the survivors in such cases is "the cause" of their survival. Such marks as the breadth of the front part of the crab's shell and the length of a bird's breastbone very probably are but "the outward and visible signs of an inward and (physiological) grace."
The marks, little peculiarities of colour and proportionate size, or some peculiar knob or horn, by which the student of species distinguishes one constant form from another, can rarely, if ever, be shown to have in themselves an active value in aiding or saving the life of the species of plant or animal. The mark or "character" is an accompaniment of a chemical, nutritional, physiological condition, and is in itself of no account. It is what is called "a correlated character." Such, for instance, is the black colour of the skin of pigs which in Virginia, U.S., are found, as stated by Darwin, not to be poisoned by a marsh plant ("the paint-root," Lachnanthes tinctoria), whilst all other coloured and colourless pigs are. The pigs which are not black develop a disease of their hoofs which rot and fall off, causing their death when they eat this special plant "the paint-root." The colour does not save the pig–it cannot correctly be called the _cause_ of the pig's survival–but is an accompaniment of the physiological quality which enables the pig to resist the poisonous herb. So, too, with white-spotted animals. They are known to breeders as being liable to diseases from which others are free. Fantail pigeons have extra vertebræ in their tails, and pouter pigeons have their vertebræ increased in number and size. But the vertebræ were never thought of and "selected" by the breeders. They only wanted a fanlike set of tail feathers in the one case, and a longer body in the other. Some varieties of feathering maintained by pigeon breeders lead to the growth of abundant feathers on the legs (as in Cochin-China fowls), and it is found that these feather-legged pigeons always have the two outer toes connected by a web of skin. If it were a stabilized wild form we should separate it as a species on account of its webbed toes, yet the real selection and survival in the hands of the breeder had nothing to do with the toes or their web, but was simply "caused" by these pigeons having feathers of "survival or selection value" in his judgment. Male white cats with blue eyes are deaf. If deafness were ever an advantage (a difficult thing to imagine), you would get a species of cat with white hair and blue eyes, and be led to distinguish the species by those characters, not by the real cause of survival, namely, deafness. Not enough is yet known of this curious and very important subject of correlation, but its bearing on the significance of "specific characters" is sufficiently indicated by what I have said.