1896.] *" [Conklin. 



1. The causes of developmeut in general are usually recognized as 

 twofold, extrinsic and intrinsic. As examples of extrinsic causes may 

 be mentioned gravit}', surface tension, light, heat, moisture, and chem- 

 ism in general ; examples of intrinsic causes are the uon-exosmosis of 

 salts from living bodies in water, the pouring of a glandular secretion 

 or the sap of plants into a cavity under high pressure, the active 

 changes in shape and position on the part of cells, assimilation, growth, 

 division, etc. There is not, however, a uniformly sharp and distinct 

 line of demarcation between these two factors of development. Phe- 

 nomena once supposed to be due entirely to intrinsic causes are now 

 known to be the result of extrinsic ones, and it is practically certain 

 that this will be found true of still other phenomena. But although it 

 is not possible to draw any hard and fast line between these two classes 

 of causes, one can, in general, recognize a very marked ditference 

 between them. Extrinsic causes may, in large part, supply the stim- 

 ulus and the energy for development, and may more or less modify its 

 course ; the intrinsic causes are of a much more complex character than 

 the extrinsic ones, they are inherent in the living matter and in large 

 part predetermine the course of developmeut. In one form or another 

 the distinction between these two classes of causes is recognized by all 

 naturalists. His calls the intrinsic causes "the law of growth," the 

 extrinsic ones the conditions under which that law operates. These 

 designations correspond, at least in part, to Prof, Cope's Anagenesis 

 and Katagenesis, and to Roux's "simple and complex components" of 

 developmental processes. 



While it is necessary to emphasize the diflerences between these 

 two classes of causes, it is not intended thereby to dogmatically assert 

 their total dift'erence in kind. It may well be that these extrinsic and 

 intrinsic causes are totally different in kind, but in our present state 

 of ignorance it would be unjustifiable to affirm it. On the other hand, 

 it would be just as unwarrantable to dogmatically affirm that there is no 

 difference in kind between these two classes of causes, and that, there- 

 fore, all vital phenomena are only the manifestations of heat, light, elec- 

 tricity, attraction, repulsion, chemism, and the like. It may be that 

 this is true, but there is as yet no sufficient evidence for it, and to at- 

 tempt, as certain dynamical and mechanical hypotheses do, to refer all 

 vital phenomena directly to such simple components as those named 

 above is practically to make impossible at present any explanation of 

 vital phenomena. "If we would advance without interruption," fays 

 Roux,* ''we must be content, for many years to come, with an analysis 

 into complex components." 



2. We need not now further concern ourselves with an explanation 

 of extrinsic causes or simple components, since this subject properly 

 belongs to chemistry and physics. If, however, we examine more 

 closely some of the intrinsic causes or complex components, we will lind 



* Wilhelm Roux, Einleitung. Archiv fiir Enturicklungsmechanik der Organismen. 



