4 • BRITISH APHIDES. 



we seem hardly yet to have placed our foot on the 

 first rungf of the ladder, which leads to its elucidation. 



Professor Allen Thomson says,* " To recognise the 

 possibility of continuous derivation in the history of 

 the origin of plants and animals doubtless has proved 

 most valuable as a working hypothesis." Thus work- 

 ing theories and fair hypotheses will ever have their 

 due weight, but both these helps must sit loosely on 

 the student of truth. He will hold fast that he has 

 received till a higher truth bids him modify his know- 

 ledge, which knowledge after all, from his circum- 

 scribed position and limited number of senses, he will 

 candidly admit can have reference to but a minute 

 section of the grand Kosmos. 



But whatever weight we may give to the modern 

 doctrine of evolution, it would seem to render more 

 and more difficult a natural and therefore a scientific 

 definition of species. The early and generally received 

 formula has undergone a change, and the hitherto 

 accepted tests of a species, as to its historic persist- 

 ence and its fertility 'hiter se, to the virtual exclusion of 

 other species as intermixing with it, noAV no longer is 

 thought to yield a decisive answer, inasmuch as such 

 proofs partake too much of a negative character. 



In the words of Professor Huxley, " the proof that 

 all members have descended from one parental stock 

 would ordinarily be considered sufficient to entitle 

 them to the rank of a physiological species, and some 

 have considered species defined as the offspring of a 

 single primitive stock." But there is always a certain 

 deviation in the off'spring from the precise character 

 of the single parent, and sometimes an exact mean 

 may be found between the two parents. Beaumur 

 long ago showed in the case of Gratio Kelleia, and 

 elsewhere more recently we have had notice of other 

 examples, t that peculiarities affecting the number of 



* Vide Address on Embryology to Brit. Assoc, 1877. 

 _t Vide 'Nature.' Oct. 18, No 448, p. 116; and " Hereditary Trans- 

 mission," 'Encyc. Britan.,' sixth series. 



