Over-development 
colouring in birds. With this phenomenon we 
shall deal more fully when speaking of animal 
colouration. There is certainly no small amount 
of evidence which seems to indicate that, from 
some cause or other, an impetus has been given 
to certain organs to develop along definite lines, 
The reduction of the number of digits in several 
mammalian families which are not nearly related 
is a case in point. This phenomenon is, as 
Cope points out, observed in Marsupials, Rodents, 
Insectivores, Carnivores, and Ungulates. He, 
being a Lamarckian, ascribes this to the in- 
herited effects of use. Wallaceians attribute it 
solely to the action of natural selection. The 
assumption of a growth-force or tendency for the 
development of one digit at the expense of the 
others, would explain the phenomenon equally 
well. And it is significant that many paleonto- 
logists are believers in some kind of a growth- 
force. In the case of certain extinct animals we 
seem to have examples of the over-development 
of organs. “ Paleontology,” writes Kellog on 
p. 275 of his Darwinism To-day, “reveals to us 
the one-time existence of animals, of groups of 
animals, and of lines of descent, which have had 
characteristics which led to extinction. The un- 
wieldiness of the giant Cretaceous reptiles, the 
fixed habit of life of the crinoids, the coiling of 
the ammonities and the nautili, the gigantic 
antlers of the Irish stag—all these are examples 
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