144 EXPERIMENTAL EVOLUTION LECT. 



With chemical investigation such as Armand 

 Gautier's, and physiological experiments such as 

 those which Vulpian originated, a great deal must 

 certainly be discovered, and herein lies a new way 

 to the investigation of the much debated question 

 of variability. Even if it does not lead us any 

 further, this line of study must bring us to this 

 much needed requisite : an answer to the oft- 

 repeated question, What is a species ? Species, 

 we are always answered, are endowed with such 

 and such characters, and when we come to look 

 at matters, we find out that two forms say of 

 Crustaceans or Rotifers are considered as speci- 

 fically distinct because the one has a few hairs 

 more than the other on this or that appendage, 

 or because the form of such or such part is more 

 elongated in one group and rather square in the 

 other. This answer is doubtless very satisfactory, 

 since so many are amply satisfied or seem to be 

 so with it ; but in future we shall certainly come 

 to define species not only by means of their external 

 anatomical characters, but also in terms of a large 

 number of physiological and chemical differences 

 which have hitherto been entirely disregarded, which 

 are but slightly apparent, and can be made quite 

 clear only by means of careful and methodical in- 

 vestigation with appropriate methods : to the mor- 



