THE STRUCTURE OF PROTOPLASM 
the cell are not, then, such as we might expect to duplicate by mixture 
in vitro. They are in some degree at least the product of the infinitely 
varied external and internal environmental changes to which the cell in 
its long evolutionary history has been exposed. 
Genetics and the Structure of Protoplasm 
It is perhaps unfair to the geneticists to consider the great contributions 
which they have made in recent years in the light of their bearing on current 
theories of protoplasmic structure. The adherents of the doctrine of unit 
factors assert repeatedly that they are quite unwilling to commit them- 
selves as to the form in which these unit factors are to be conceived as 
existing in the germ plasm. We are told that they are merely presenting 
facts as to the behavior of visible characters in breeding experiments and 
that we may imagine any sort of representation of these factors in the germ 
plasm which we please. And yet views so widely held and so stimulating 
of research as the factorial hypotheses are certain to influence strongly at 
least our a priori conceptions of the structure of the germ plasm and of 
protoplasm in general. 
The serial arrangement of such factors in the chromosomes seems to be 
involved in that parallelism between the data as to chromosome reduction 
and the segregation of factors which has afforded such strong support for 
the whole Mendelian theory. We may note in passing that Trow points 
out regarding the evidence of serial arrangement derived from linkage that 
the numerical data used as the basis of the assumption of a serial order of 
the factors constitute "a type of representation common to every set of 
phenomena which can be expressed as percentages." 
Still, it may be noted further that if we are not to regard the chromo- 
somes as so many chains of factorial beads there is no other hypothesis 
which has any recognized standing at present with either cytologists or 
geneticists. The development of the conception of the cell as a polyphase 
colloidal system seems to point in another direction, but it has led to no 
very definite ideas as to the way in which the characters of the adult many- 
celled organism are represented in the germ plasm. It is doubtless true, as 
is so strongly felt by the newer epigeneticists like Greil, that the idea of a 
representation of characters of a many-celled organism, especially those 
due to the inter-relations of the cells, in the chromosomes cloaks a vast 
amount of obscurity. These latter biogenetic characters must certainly 
be put in a different category from the characters which Detto has called 
metidentical and whose inheritance offers relatively little theoretic difficulty. 
It is certainly worth while to consider most carefully these conceptions 
based on the facts of breeding from the viewpoint of their influence on 
theories of protoplasmic structure. The day is past for the explanation of 
vital phenomena by the assumption of units or particles endowed with 
properties which explain their assigned functions. Nowhere is the need 
