EVOLUTION 



377 



appears in a variety of some common species. We may- 

 take an example from the springtails of the genus Achorutes 

 mentioned in a previous chapter (pp. 264-6). The species 

 of Achorutes have, as a common character, a pair of spines 

 (Fig. 81, tf) at the tail end of the body, longer in some species, 

 shorter in others. Other genera of the same family have no 

 spines, or three, or four, as a distinguishing feature. Yet in 

 several species of Achorutes a few examples out of a large 

 number may have no spines at all, or there may be a third 

 median spine (Fig. 81, b) developed between those of the 

 normal pair. Such facts are suggestive of the possibility 

 of rather quick change in the history of a race ; but it must 

 be remembered that for a new genus to arise in this way, 



Fig. 81. — Tail-end of Springtail (Achorutes longispimis) Spitsbergen. 

 a, normal two-spined specimen ; b, three-spined variety. X 300. 



further distinctive characters would be necessary and the 

 creatures possessing them would need to '* breed true." 



In the course of the early controversies that raged during 

 the last century around the evolution problem, it was often 

 urged by opponents of Darwin that incipient species could 

 never be established, as they would be " swamped by 

 intercrossing." The segregation brought about through 

 Mendelian inheritance disposes of this objection, and in the 

 case of various insects — Amphidasys and Drosophila, for 

 example — already used in illustration, we see how the new 

 form is preserved in a pure strain. Such a result may be 

 regarded as a kind of germinal isolation ; the importance of 

 various types of isolation in the fixation of systematic 

 differences was emphasised by G. J. Romanes (1897), and as 



