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shows this most clearly — shows that they continue during very long periods 

 very gradually, and so cause a considerable change in the form of the animal 

 species in which they occur. 



The form, for instance, of a hornless Sphingidae- caterpillar like Deilephila 

 Vespertilio F. differs so markedly from its original stock which existed perhaps 

 many thousand years ago, and was provided with a long, mobile tail horn 

 possibly covered with long poisonous prickles, that it would certainly be taken 

 as a specific distinction, if it were found at the same time, and in a complete 

 insect, not a caterpillar. And it is possible for several of these processes to 

 be developing simultaneously in the same species; this is the case in the 

 Rhopalocera, with the diminishing of the wings, the atrophication of the fore- 

 legs and the colour-evolution ; their combined influence can greatly change an 

 animal form in the course of time. Seeing that there is no doubt, that these 

 evolutionary changes are brought about very gradually, is it probable that 

 other changes of the same nature should take place quite suddenly? Is it 

 conceivable, that the nature of this great natural phenomenon Evolution — for that 

 is the point in question — could be of such a different nature ? I mean, of course, 

 in reality; for it is certainly possible, that it may appear so to our imperfect 

 observation; but this is not the sense in which it is meant by the theory we are 

 discussing. In all my many observations of evolutionary change, I have never seen 

 it happen suddenly. It is true that the moment at which it begins in a particular 

 individual is to some extent sudden, but neither is this what is meant by the 

 theory ; when the same process of change is seen to arise in different individuals, 

 in the one earlier and more markedly than in the other, and then to extend 

 in the same unequal manner, it is a clear demonstration of the gradual 

 development of one same process. And it is always of such process that these 

 changes in form are the expression. The few examples of mutation observed 

 in animals that I have seen mentioned in different works, have always given 

 me the impression of being based upon very superficial and doubtful observations; 

 I never found the least attempt to explain these observations in any other way 

 and I can attach no value to them. As regards the really accurate investigation 

 by E. I. BouviER of the changes observed in Crustacea, it refers to a process 

 of change of form in a group of Crustacea of precisely the same nature as 

 that which I have made a study of in the Rhopalocera, and this entomologist 

 quite rightly recognises it as due to evolutionary changes, and calls them 

 "mtitafions evohiiivcs" . But not understanding the true nature of them, he sees, 

 in the great variety of ways in which the process develops in different individuals, 

 not the gradual course of the process, but so many sudden mutations. This 

 is not the interpretation given by the mutation theory however; the formation 



