VARIATION AND DIFFERENTIATION IN CERATOPHYLLUM. 



By Raymond Pearl, 



With the assistance of Olive M. Pepper and Florencb J. Haqle. 



INTRODUCTION. 



The purpose with which this investigation was undertaken was to 

 attempt to work out as exactly and completely as possible for a particular 

 organism the laws according to which post-embryonic differentiation and 

 growth occur. In higher plants and animals the phenomena of growth 

 are accompanied by phenomena of differentiation. The two processes 

 go along together until finally the adult condition is reached, in which 

 the organism has attained not only a certain size, but also a complex 

 condition of differentiation of parts or organs. In studies of variation 

 it has been the usual— though by no means invariable— custom to take a 

 particular character or set of characters found in the adult as something 

 given, and then proceed, by processes now becoming well known, to 

 investigate the nature and degree of the variation exhibited in these 

 characters. But since the condition of the adult organism is, with 

 respect to every character, the result of a process of gradual develop- 

 ment and growth, it is clear that in order to gain anything approaching 

 a satisfactory analysis from the biological standpoint, we can not take 

 the adult structure as something given, but must investigate the laws 

 according to which the morphogenetic processes concerned in its pro- 

 duction operate. There is no doubt that the problem presented by the 

 phenomena of morphogenesis is one of the most fundamental in biology. 

 Just at present we are witnessing a period of remarkable activity in the 

 investigation of the problems of heredity, and we are told that here 

 lies the way to follow when in search of biological truth. No one can 

 doubt either the immense importance of a determination of the laws of 

 inheritance, or the value of the contributions to a knowledge of these 

 laws which have been made during the last few years, both by the 

 biometricians and by those working along Mendelian lines. Yet one 

 ventures to think that it is of equal importance that our knowledge of 

 the laws of morphogenesis be extended. In the zeal for the new thing 

 represented by Mendelian inheritance and the phenomena of mutation 

 there is a tendency to overlook the fundamental significance for our 

 whole outlook on the broader problems of biology, of the results which 



