INTRODUCTION 



influence the forms of the organic world, which indeed 

 primarily condition them, many characters of this organic 

 w^orld must exist \vhich have nothing whatever to do with 

 utility. 



It was a fTiori to be expected that the result of niv 

 investigation would relate principally to such characters, 

 hitherto but little considered. 



If we could know all the natural laws which have operated 

 in the evolution, and which operate in the existence of a sin<de 

 animal or a single plant, w^e should understand the laws of 

 the organic world altogether. 



On this ground alone the biologist may look for a rich 

 reward in giving himself unreservedly for once to the study 

 of a single living being, in order to penetrate into its nature 

 as deeply as possible. The single chosen object soon tells 

 him more than all other animals or plants together whicli he 

 has observed more superficially. For the more an investigator 

 devotes himself to a fruitful object, the richer it appears to his 

 eyes, the more it shows new properties, the more it acquires 

 living interest and importance, and the more do all its 

 characters and the relations of its life show themselves to be 

 governed by law. 



The unreserved study of a single species of animal has 

 led me to the discovery of a whole series of laws, which the 

 extension of the investigation to other species showed to hold 

 good generally. 



As far back as the beginning of the eighth decade of the 

 present century, I had turned my attention to the wall-lizard, 

 wdiose remarkable variability is well known, as the starting- 

 point of my investigation. The circumstance that in tlie 

 spring of 1872 I became acquainted with the dark blue wall- 

 lizard, described by me as Lacerta muralis ccerulea, on one of 

 the Faraglione rocks at Capri determined my final decision. 



In this form of the wall -lizard I had found an animal 



