24 INTRODUCTION. 



processes of their development from the egg. For although the 

 morphological section of animal biology teaches with much 

 probability that this species or that organ has undergone this or 

 that course of modification in the animal series, and that 

 in the process of modification it has passed through a whole 

 series of various forms, still it is only physiological research 

 that can elucidate the necessity for their existence by revealing 

 their causative conditions. 



