INTRODUCTION 



which might be with equal justice described as species or 

 variety, so much does it differ from the original form. Thus, 

 at the conclusion of the paper I published on this lizard, 1 I 

 was able to say : " The remark of some opponents of Darwin- 

 ism that no one has yet observed the conversion of a variety 

 into a species is not, I grant, contradicted by the existence of 

 the blue wall-lizard, for it makes on the adherents of the 

 theory a demand similar to this : we want to hear the grass grow. 

 Besides, every naturalist will always be at liberty, within cer- 

 tain limits, to understand by a ' species ' what he will. But an 

 instance is afforded in this animal of undoubted natural race- 

 production, which has evidently occurred in a relatively short 

 period of time, and this ought to prevent any opposition, which 

 is not expressly one of principle, from attaching any import- 

 ance to the argument above quoted." 



The problem of finding the causes of the modifications 

 which this remarkable variety had undergone led me im- 

 mediately into the midst of the inquiries which I had already 

 proposed to myself. The result of my researches, which were 

 extended to various classes of animals, was the recognition of 

 the dominion of laws in the process of variation, not only in 

 the lizard, but also in the most diverse tribes of the animal 

 kingdom : these laws holding firstly in the variations of 

 marking, previously regarded as quite indifferent, unimportant, 

 and fortuitous, but also applying to other characters. I was 

 able to demonstrate that variation everywhere takes place in 

 quite definite directions which are few in number, and I was 

 able on the basis of my observations to put forward the view 

 that the causes which lead to the formation of new characters 

 in organisms, and in the last result to their evolution, consist 

 essentially in the chemico-physiological interaction between 

 the material composition of the body and external influences. 



1 Zoologische Studien auf Capri, ii. Lacerta muralis ccerv.lea, Leipzig, 

 Engelmann, 1874, 



