ORIGIN OF ADAPTATIONS AND OF SPECIES 24! 



mulation of such advantageous variations as already exist. 

 Variation, then, is the basis of natural selection. Though the 

 question of the origin of variations is still unsettled, the fact 

 of their occurrence in a manner sufficient for the purposes of 

 natural selection is beyond dispute. No two individuals of a 

 species are ever exactly alike in structure or behavior, and 

 their differences furnish the material for the operation of 

 natural selection. 



Two classes of variations are distinguished on the basis of 

 the amount of variation : (i) continuous (individual) varia- 

 tions, of small extent, intergrading with one another and with 

 the typical form; and (2) discontinuous variations (sports), 

 or considerable and isolated departures from the normal con- 

 dition. Furthermore, variations of either class are adaptive 

 or unadaptive, the latter kind being either harmful or simply 

 neutral. 



Origin of Adaptive Variations. Natural selection, as 

 was said, does not begin to operate until useful variations are 

 already in existence; and the origin of these primary adaptive 

 variations is a question quite distinct from that of their sub- 

 sequent preservation and accumulation by natural selection. 



That all adaptive variations are due to the response of pro- 

 toplasm to environmental influences (using the term " envi- 

 ronment " in its widest sense), it goes without saying. These 

 variations are, however, either direct or indirect. Direct 

 variations, appearing first in the soma, or body, of the organ- 

 ism, are termed soinatogenic; indirect variations, apparently 

 spontaneous, and due immediately to the germ cells, are termed 

 blastogenic. Weismann places somatogenic variations, ac- 

 cording to their origin, into three categories : ( I ) injuries, 

 (2) functional variations, and (3) variations depending on 

 the so-called " influences of environment," these influences 

 being mainly climatic. These three kinds will receive brief 

 consideration. 



Injuries. There appears to be no good evidence that in- 

 juries or mutilations can be transmitted. Nearly all the ex- 

 '7 



