THEORY OF THE DEVELOPMENT OF ORGANISMS 105 



in the following example: it depends on their 

 nutrition whether certain trees shall bear foliage 

 or flowers ; while in an unpropitious climate many 

 plants refuse to bear flowers at all, but content 

 themselves with vegetative reproduction.' 



This principle indicates the path along which 

 explanation of the differentiation of cells is to be 

 sought. Although in no single case is it yet 

 possible to refer a known action to its appropriate 

 cause in other words, to show a definite stimulus 

 producing a definite reaction upon the rudiment 

 this failure is not to be attributed to error in the 

 principle. It is the natural result of the enormous 

 difficulties besetting an attempt to understand 

 the highly involved events of development. We 

 can only ask whether or no our general prin- 

 ciple is harmonious with the facts displayed in 

 nature. 



In the following pages I shall try to develop 

 this view, taking, as formerly, a few instances. I 

 shall now proceed further with suggestions I made 

 in my treatise on Old and New Theories of Develop- 

 ment. I start from the conception that the ovum 

 is an organism that multiplies by division into 

 numerous organisms like itself. I shall explain the 

 gradual, progressive organisation of the whole 

 organism as due to the influences upon each other 

 of these numerous elementary organisms in each 

 stage of the development. I cannot regard th^ 

 development of any creature as a mosaic work. I 

 hold that all the parts develop in connection with 

 each other, the development of each part always 



