18G4.] 411 



species of oak causing the difference in the nature of the galls, and the 

 poisonous matter producing the gall being the same in both forms. 

 But there are so many Cj/iu'ps that produce exactly the same kind of 

 gall on different species of oak, that it would seem that these two forms 

 must be true Phytophagic Species, each generating a peculiar kind of 

 gall-producing poison, and each with such internal diff'erences as to 

 cau.«e them to generate secretions with such very different properties. 



That there may be no possible mistake, it may be as well to say here, 

 that the difference between what I call a Phytophagic Variety and 

 what I call a Phytophagic Species is simply this : — The former habit- 

 ually intercross with the normal race, the latter do not. Of course 

 there must often be cases, where the fact of their habitually intercross- 

 ing or not so intercrossing is doubtful or cannot be satisfactorily in- 

 ferred or ascertained, and allowing that the former category sometimes 

 gradually in a long period of time merges into the latter, there must 

 be occasionally intermediate categories. Still this is no reason why we 

 .should doubt or deny the existence of the categories themselves. Boy- 

 hood is one thing, and manhood is another thing; but there are inter- 

 mediate periods when it is difficult to say whether the individual in 

 question is boy or man. Yet it would be strange logic to argue that, 

 on that account, boyhood was the same thing as manhood. 



It must be obvious to every one, that it is impossible to trace the 

 gradual formation of what T have called a Phytophagic Species in time, 

 because by the supposition it requires very long periods of time for its 

 development, and Natural History as a science is only a hundred years 

 old. But if we are able to discover the several steps in the above- 

 described process, not indeed in one and the same species, but in dif- 

 ferent species, and can thus trace an uninterrupted series from the 

 first origin of the Phytophagic Variety to the full development of the 

 Phytophagic Species, to all minds unbiassed by preconceived theories 

 the proof will be complete. In any case, assuming the truth of the 

 above Theory, this is the only possible way, in which for ages to come 

 that truth can be demonstrated. For any one therefore to refuse to 

 admit the validity of such proof, is equivalent to saying, that, even if 

 the Theory is true, he will not believe it on the best possible evidence. 



Investigations of this character require laborious and tedious experi- 

 ments in the closet, and habits of patient observation and industry in 



