338 cook 



inbreeding, and this debilitating process has been continued 

 with white mice ever since the original specimens were caged, 

 while gray mice have mostly remained at liberty until needed 

 for breeding experiments. To overlook these historical differ- 

 ences is to neglect factors of known significance for those 

 of purely hypothetical meaning. 



A second series of pertinent facts commonly ignored is the 

 frequent and perhaps general dominance or prepotency of muta- 

 tions when bred upon their own immediate blood-relations. 

 Commercial white mice are a long standing breed, with no 

 close and equally inbred gray relatives. To test prepotency 

 fairly a new mutation would be required. There are numerous 

 instances in literature, but experimenters naturally attach special 

 importance to what happens in their own cages. 



For a third experiment which might afford conclusive evi- 

 dence on the pure germ-cell theory, some of the more recently 

 developed varieties of mice might serve. If two varieties of 

 independent origin which had been crossed separately with mice 

 of the ancestral type and found to mendelize, were then crossed 

 with each other and found to revert to the parental type, experi- 

 mentalists might admit that the doctrine of pure germ-cells had 

 been definitely disproven. The mice which in the Mendel 

 experiments had produced pure white, yellow or black germ- 

 cells would later have produced gray germ-cells. And yet this 

 possibility in crosses of selected domesticated varieties has been 

 known since the time of Darwin's experiments with pigeons. 



The arrangement of the chromatin granules into chromo- 

 somes, to which so much importance is ascribed, is a very tempo- 

 rary phenomenon. The chromosomes do not appear to retain 

 their separate identity either during sexual fusion (mitapsis) or 

 during vegetative growth, when the activities of the cells are 

 bringing to expression the qualities which have been transmitted 

 through the gametes. The diversity in number of chromo- 

 somes in closely allied species, or even in the same species, also 

 tends to weaken our faith in the idea that chromosomes as such, 

 or as character groups, play a very definite or determining part 

 as governors of the form of the organic structure of the indi- 

 vidual plant or animal. The chromosomes may prove, after 



