PROGRESSING DIVERGENCE OF CHARACTERS. 149 



by the amount of difference in their morphological and physiological 

 characteristics. 



This conclusion of Darwin's, which extends the result of natural 

 selection from the production of variety to that of species, is ob- 

 stinately and often bitterly opposed by those who subordinate the 

 phenomena of nature to traditional ideas. 



Even if they do not deny the facts of variability, and even admit 

 the inriuence of natural selection on the formation of natural varieties, 

 they yet continue true to the belief that there is an absolute separa- 

 tion between species and race-variety. As a matter of fact, however, 

 we are not in a position to draw such a line of separation. Neither 

 the quality of the distinctive characteristics nor the results of cross- 

 ing afford us a distinctive criterion between species and variety. 

 The fact, however, that we are not able to give any satisfactory defini- 

 tion of the conception of species, precisely because we, are unable clearly 

 to distinguish between species and variety, adds so much the more 

 weight to Darwin's argument, since neither the variability of the 

 organism and the struggle for existence nor the great antiquity of 

 life upon the globe can be contested. 



The variability of forms is a firmly established fact ; so, too, is the 

 .struggle for existence. Now if we add the operations of natural 

 selection to these two factors, we are able to understand the origin 

 of varieties. If we imagine the same process which has led to the 

 formation of varieties continued through a greater number of genera- 

 tions and effective through a longer period of time and we are the 

 more justified in making use of these long periods of time, since 

 with their help astronomy and geology have been enabled to explain 

 many phenomena the diverging characteristics will become more and 

 more marked, and will at last gain the value of distinctive species- 

 characters. 



In still greater periods of time the species will become so far 

 separated from one another by the simultaneous disappearance of 

 the intermediate forms that they will represent different genera. 

 Accordingly the greater differences of organization which are ex- 

 pressed in the higher divisions of the system, such as orders and 

 sub-orders, etc., require a longer interval of time for their accom- 

 plishment, and an extinction of a greater number of intermediate 

 forms. Finally, the different ancestral forms of the classes of a 

 group may be referred to a common starting-point ; and since the 

 different groups of animals are connected by many intermediate forms, 

 the number of the ancestral forms becomes much reduced. 



