THE GERM-PLASM THEORY 355 



all other theories of development founded on a similar basis, that 

 they could not be accepted even if they were able to offer a workable 

 explanation of the development of the individual, and for this 

 reason, that ontogeny is not an isolated phenomenon which can be 

 interpreted without reference to the whole evolution of the living 

 world, for it is most intimately associated with this, being indeed 

 a piece of it, having, as we shall see, arisen from it, and, furthermore, 

 preparing for its continued progress. Ontogeny must he explained in 

 harmony with phylogeny and on the same principles. The assumption 

 of a germ-plasm without primary constituents, or of a completely 

 homogeneous germ-plasm, as Herbert Spencer maintained, is irre- 

 concilable with this, for, as will be seen, it contradicts certain facts 

 of inheritance and variation. Therefore all theories founded on 

 this assumption must be rejected. 



There is another and, I believe, weighty consideration which 

 forbids us to assume a germ-substance without primary constituents. 

 I shall return to this later, but in the meantime I wish to build up 

 more completely my own ' germ-plasm ' theory. 



I assume that the germ-plasm consists of a large number of 

 different living parts, each of which stands in a definite relation to 

 particular cells or kinds of cells in the organism to be developed, that 

 is, they are ' primary constituents ' in the sense that their co- 

 operation in the production of a particular part of the organism is 

 indispensable, the part being determined both as to its existence and 

 its nature by the predestined particles of the germ-plasm. I therefore 

 call these last Determinants (BestiTnmungsstucke), and the parts of the 

 complete organism which they determine Determinates, or hereditary 

 parts. 



It is easy to show on what basis this assumption rests; the 

 phenomena of inheritance taken in conjunction with those of variation 

 seem to me to compel us to it. We know that all the parts of an organism 

 are variable, and that in one individual the same part may be larger, 

 in another smaller. Not all variations are transmissible, but many of 

 them, and some very minute ones, are. Thus, for instance, in many 

 human families there occurs a small pit, hardly as large as the head 

 of a pin, in the skin of the ear, whose transmission I have observed 

 from the grandmother to the son and to several grandchildren. In 

 such a case there must be a minute something in the germ-plasm, 

 not present in that of other human beings, which causes the 

 origin, in the course of development, of this little abnormality in 

 the skin. 



There are human families in which individuals occur repeatedly, 



