84 Evolution and Adaptation 



Conclusions 



In the light of the preceding discussion concerning the 

 evidence in favor of the transmutation theory, we may now 

 proceed to sum up our general conclusions, and at the same 

 time discuss some further possibilities in regard to the 

 descent theory. 



The most widely accepted view in regard to the theory of 

 organic evolution is that which looks upon the resemblances 

 between the members of a group as due to their common 

 descent from one original species that has broken up, as it 

 were, into a number of new forms. Strictly applied, this 

 means that all the vertebrates have come from one original 

 species, all the mollusks from another, the echinoderms from 

 a third, etc. Even farther back there may have been a com- 

 mon ancestral species for any two of the large groups, as, 

 for example, the annelids and the mollusks ; and if the re- 

 lationship of all the many-celled forms be looked upon as 

 probable, then they too have originated from one ancestral 

 species. 



Many zoologists appear to hesitate to apply strictly this 

 fundamental idea contained in the transmutation theory, be- 

 cause, perhaps, they feel that it does not fit in with their gen- 

 eral experience of living forms. Yet there can be no doubt 

 that it is the primary conception of the transmutation 

 theory. This is, however, not the whole question, for we 

 must further consider the number of individuals of a species 

 that are involved. 



In some species there are smaller groups of individuals 

 that are more like one another than like other individuals of 

 the same species. Such groups are called varieties, and are 

 often associated with certain localities, or with a special 

 environment. In the latter case they are called local varie- 

 ties. Some of these appear to breed true, not only when 

 kept under the same conditions, but even when transferred 



