2 BRITISH APHIDES. 



forms become accurately present, and sliades of dif- 

 ference become cognizable, wliicli no descrijotion can 

 make intelligible. 



In the next place something perhaps should be said 

 to meet the disappointment of those who might look 

 through the following pages for some substantial help 

 to the modern doctrine of evolution. Possibly it 

 might be thought that something definite should be 

 made out from a complete examination of the almost 

 exceptional group Aphis, in which Parthenogenesis is 

 the rule, and Dimorphism by no means uncommon. 



But, however much such proof might be desirable, or 

 acceptable to many, the author's observation hitherto 

 has not led him up to phenomena indicating sure 

 paths towards the transmutation of species, although 

 climatal influence has a certain effect on variation. 



A great thinker has said '^Wise man w^as he who 

 counselled that speculation should have free course, 

 and look fearlessly towards the thirty-two points of 

 the compass, whithersoever and howsoever it listed."* 



Certainly the How, the Why, and the Whither, are 

 questions not unasked by the thoughtful of the pre- 

 sent day ; but we are warned by another no mean 

 authority " lest we do injury to the future by a too 

 free use of hypothesis." Exact science is securely hiiilt 

 on experiment and rests not on speculation. Every 

 step proved is a kind of Archimedean " ttov arw,"" effi- 

 cient for a fresh start, and, not being a mere opinion, 

 it is different in character from that which has not 

 been definitely determined. 



Professor Virchow has well said " that each essen- 

 tially new truth of the former kind must necessarily 

 influence the whole conception of man — the method 

 of thinking." 



Again, " Human knowledge is but piecework, and 

 all who call themselves naturalists only possess pieces 

 of natural science." 



The outer world from necessity must accept the 



* Vide ' Sartor resartus.' 



