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 palaeonto- 

 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. " Palaeontology, " 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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