118 EVOLUTION AND GENETICS 



change of response. The persistence of a response removed from 

 the normal is a modification of the internal environment, and as 

 such is capable of initiating further modifications in the organism, 

 so that, whatever may be their source, modifications established 

 in an entire species or in a large majority of the individuals of a 

 species are at least a potential stimulus to permanent change. The 

 possible behaviour of such factors is another of the problems of 

 evolutionary theory. 



Summary. We have seen that relationship implies not merely 

 resemblance, but similarity of origin, and that consequently the 

 various degrees of relationship which are found among the existing 

 species of animals point to common derivations of varying remote- 

 ness. The details of relationship indicate definite successions 

 which represent the probable ancestry of different groups. Change 

 of individuals is possible in response to change of environment, the 

 environment is known to change, and single species exist even now 

 which are capable of responding in ways fundamentally different. 

 It is especially significant that while existing organisms show the 

 possibility of merely analogous structures meeting the same need, 

 in many cases the same fundamental structure is modified to meet 

 many needs. Evidences of the past occurrence and the present 

 possibility of evolution are abundant, and the factors mentioned 

 of variation, adaptation and environment furnish the materials 

 for further inquiry into the processes involved. 



