Hyatt.] 240 [December 16, 



becomes possible to compare the life of the individual with the life of 

 the group to which it belongs, the period of growth and develop- 

 ment to the period of the progressive evolution of new forms, and the 

 period of old age with its retrograde metamorphoses to the period of 

 decline during which retrogressive forms are evolved. This compar- 

 ison and the facts noted above enable us to attribute the parallel mod- 

 ifications of forms, whether occurring during the progressive or 

 declining period in the existence of a group, to the direct influence 

 of the environment. 



Besides these characteristic forms and structural parts which are 

 parallel, there are many others in each group not classified under the 

 head of similarities but under that of differences, in so far as they 

 distinguish the groups from each other. These may be often followed 

 back to varieties of one species, showing that certain varieties have 

 given rise to the groups. These varieties are few as compared with 

 the whole number of varieties traceable in these original aricestral 

 species. 



Thus it seems clear, that these varieties must have had certain 

 advantageous peculiarities enabling them to survive the climatic or 

 geological changes, which destroyed the weaker descendants of the 

 same stock, and that these peculiarities rendered them capable of 

 perpetuating their race until they arose into a group or series of 

 genetically connected forms. 



Unless the Darwinian law of natural selection, or the survival of 

 the fittest, does apply to the perpetuation of these structural differ- 

 ences which distinguish groups from each other I am entirely at loss 

 in my attempts to account for them. I here carefully guard against 

 attributing the origin of these differences to the law of natural selec- 

 tion, but limit its action strictly to the modification of the structural 

 differences which tend to appear first in the varieties and then by in- 

 heritance in larger and larger groups and at earlier and earlier stages 

 in the life of the individual. 



It may also be shown by Cope's law of the origination of differ- 

 ences by growth that the origin of these differences probably lies in 

 some law of growth under the influence of the physical surroundings, 

 supply and kind of food, climate, etc. Thus they may be said to 

 be due to growth modified and directed by the Darwinian law of 

 natural selection, both of these being directly subject to the influence 

 of the environment, or the sum of all the physical influences brought 

 to bear upon the organization. 





