222 Animal Life and Intelligence. 



something additional have to pass through more numerous 

 stages than their ancestors ; and those which lose some- 

 thing pass through fewer stages than their ancestors ; and 

 these processes are expressed by the terms ' acceleration ' 

 and ' retardation.' " * 



It is clear, however, that we have here something more 

 than acceleration and retardation of development in the 

 ordinary sense of these words. It would be, therefore, more 

 convenient to use the term " acceleration " for the con- 

 densation of the same series of developmental changes into a 

 shorter period of time ; "retardation " for the lengthening of 

 the period in which the same series of changes are effected ; 

 and " arrested development " for those cases in which the 

 young are born in an immature or embryonic condition. 

 Whether there is any distinct tendency, worthy of formu- 

 lation as a law, for organisms to acquire, as a result of 

 protracted embryonic development, definite characteristics 

 which their ancestors did not possess, I think very question- 

 able. If so, this will fall under the head of the origin of 

 variations. 



That acceleration, in the sense in which I have used 

 the term, does occur as a variation is well known. "With 

 our highly improved breeds of all kinds," says Darwin,t 

 " the periods of maturity and reproduction have advanced 

 with respect to the age of the animal ; and in correspondence 

 with this, the teeth are now developed earlier than formerly, 

 so that, to the surprise of agriculturalists, the ancient rules 

 for judging of the age of an animal by the state of its teeth 

 are no longer trustworthy." " Disease is apt to come on 

 earlier in the child than in the parent ; the exceptions in 

 the other direction being very much rarer. "J Professor 

 Weismann contends that the time of reproduction has been 

 accelerated through natural selection, since the shorter the 

 time before reproduction, the less the number of possible 

 accidents. We may, perhaps, see in the curious cases of 



* Cope, " Origin of the Fittest, " pp. 226, 125, and 297. 



t "Animals and Plants under Domestication," vol. ii. p. 313. 



I Ibid. p. 56. 



