EVOLUTION AND NATURAL SELECTION 13 



selection accumulating to any extent sUght modifications of 

 instinct which are in any way useful." 



A remarkable example of perfected instinct is shown in one 

 of the hymenopterous insects known as Thalessa (see illustra- 

 tion). This is an ichneumon fly having the end of the body pro- 

 vided with a long ovipositor with a chisel-hke extremity. AYith 

 this instrument she bores into the sohd wood of a tree and with 

 great precision strikes the burrow of the Tremex larva. After 

 reaching the burrow she deposits an egg, which, soon after hatch- 

 ing into a larva, crawls along the burrow and attaches itself 

 and feeds on the body of its victim. 



Formation of New Species 



It has been lately maintained by DeVries that species 

 can come into existence within the space of a man's life- 

 time. This has been observed to take place in the evening 

 primrose in a state of nature. Darwin maintained that 

 natural selection acts by accumulating slight successive favor- 

 able variations. It can produce no great or sudden modifi- 

 cation. But, according to DeVries, species have not arisen 

 through gradual selection continued through hundreds of 

 thousands of years, but through sudden, though small, trans- 

 mutations, or steps. 



In contrast ^-ith fluctuating variations, which are changes 

 in a Hnear direction, the transformations called mutations 

 diverge in new directions without apparent definite direction. 

 MacDougal, after considerable experimentation, has gone so 

 far as to say: "Having ascertained at what time in the Ufe 

 period of the individual mutations occur, I liaAC been so 

 fortunate as to secure results demonstrating that mutations 

 may be induced in a species not hitherto active in this 

 respect, and that it is possible to call out new species 

 by the intervention of external agents during the critical 

 period." ^ 



Of DeVries' views on this subject Wliitman says: "The 

 so-called mutations of the primrose are undoubted facts, but 

 two leading c-[uestions remain to be answered. First, are these 

 ^"Heredity and the Origin of Species," 1905, p. 31. 



