142 ON VARIABILITY AND ADAPTATION 



matter. In other words, difficult as it is to conceive or picture 

 to oneself the details of the process, growth is essentially a process 

 of conversion, a chemical process, and any adequate theory bearing 

 on the phenomena of growth must primarily be along chemical lines. 

 We are ignorant of what it is in the structure of living matter 

 that gives it those properties ; we are, if possible, more ignorant 

 of the physics of the process of growth, of the nature of the 

 force which, acting upon or inherent in the constitution of living 

 matter, leads to this continuous process of assimilation and 

 conversion, and in our ignorance we are unable to separate the 

 chemistry and the physics of the process ; we must, that is, 

 regard growth as a property of living matter. We must also for 

 present purposes speak of the matter which is essential to and 

 directly concerned in the activity of any one species or individual 

 as a single substance which, following Nageli, we can refer to 

 as " idioplasm," and our conception of the individual or of the 

 separate cell units forming that individual must be that in each 

 we have to deal with two constituents, the idioplasm, or essential 

 and directive living matter, and the cytoplasm, which is in the 

 strictest sense non-living or certainly unable to exhibit the whole 

 series of vital properties apart from the idioplasm, and which 

 consists of various formed elements developed and influenced 

 by the controlling idioplasm, intimately connected therewith, it is 

 true, but at the same time not an essential part of the same — 

 the cytoplasm varying in its composition and nature under 

 varying conditions which affect the idioplasm, the idioplasm 

 under all conditions retaining certain cardinal properties. 



That the constitution of the idioplasm is not absolutely but 

 only relatively constant has also to be assumed. We are bound 

 to recognize that it is capable of undergoing modifications 

 within certain limits without loss of its cardinal properties, 

 and this from the following reasons : Admitting, as we must, 

 that the highest forms of animal and vegetable life have been 

 evolved from the very simplest, that there has been an unbroken 

 line of development of living forms from the simplest unicellular 

 to the most complex multicellular ; admitting also that in every 

 act of reproduction, however simple or however complicated, 

 the direct conveyance of the living matter of the parent into 

 the offspring is to be demonstrated, that, in other words, the 

 idioplasm of the primal living being has been continued on to 



