INTRODUCTION 3 



is of fundamental importance as well for classification as for the causal 

 explanation of the animal form. The developmental stages of man show 

 definite regular agreements, not only with the structure of the adult human 

 being, which in and of itself would be intelligible, but also with the struc- 

 ture of lower vertebrates, and even with many of the still lower inverte- 

 brate groups. 



Physiology. In the same way as the morphologist studies the 

 structure, the physiologist studies the vital phenomena of animals and the 

 functions of their organs. Formerly life was regarded as the expression 

 of a special vital force peculiar to organisms, and any attempt at a logical 

 explanation of the vital processes was thereby renounced. Most modern 

 physiologists have abandoned this theory of a vital force; they have 

 attempted to explain life as the summation of extremely complicated 

 chemico-physical processes, and thus to apply to the organic world those 

 principles which prevail in the inorganic realm. 



Developmental Physiology ("Entwicklungsmechanik"). Since 

 each organism is the product of its development; since, further, the 

 development represents the summation of most complicated vital processes, 

 the explanation of the organic bodily form is, therefore, in ultimate anal- 

 ysis a physiological problem; a problem whose solution lies still in the in- 

 definitely distant future. It has to explain how the apparently simple 

 fertilized egg is converted into the complicated adult organism with its 

 many organs regularly arranged. The potentiality of the complexity of 

 the adult must be contained in the egg. But it is still a matter of dispute 

 as to how this potentiality is conditioned: whether as a mosaic of minute 

 particles, each corresponding to a peculiarity of the adult organism, or as a 

 substance of simpler structure, in which complexity only appears in the 

 course of development. We can proceed experimentally in such a way 

 that the conditions of development are artifically altered, and the results 

 may be compared with the normal processes. It is also possible to study 

 the modifications which one and the same developmental process under- 

 goes in different species, modifications which are dependent upon the life 

 conditions of the animals and of their young. Then, too, there are ex- 

 periments of the same kind performed by nature and which have the same 

 informing value as the artificially arranged conditions. Such researches 

 have accomplished much in the last decade and have resulted in a 

 deeper understanding of the developmental processes. 



The potentialities contained in the fertilized egg are the hereditary 

 elements which are transmitted from the parent to the next generation 

 and which result in the resemblance of the offspring to the parents. The 

 study of these elements and the way in which they are transmitted from 



