HEORIES OF WEISMANN AND DE VRIES 397 
But while sexual reproduction may be evoked to explain 
the origin of variation in higher animals, Weismann thought 
it was not applicable to the lower ones, and he found himself 
driven to assume that variation in single-celled organisms is 
owing to the direct influence of environment upon them, 
and thus he had an awkward assumption of variations arising 
in a different manner in the higher and in the simplest organ- 
isms. If I correctly understand his present position, the ~™ 
conception of variation as due to the direct influence of 
environment is being surrendered in favor of the action of 
germinal selection among the simplest organisms. 
Extension of the Principle of Natural Selection.—These 
variations, once started, will be fostered by natural selection 
provided they are of advantage to the organism in its struggle 
for existence. It should be pointed out that Weismann is a 
consistent Darwinian; he not only adopts the principle of 
natural selection, but he extends the field of its operation 
from externals to the internal parts of the germinal elements. 
“Roux and others have elaborated the idea of a struggle 
of the parts within the organism, and of a corresponding 
intra-selection; . . . but Weismann, after his manner, has 
carried the selection-idea a step farther, and has pictured 
the struggle among the determining elements of the germ- 
cell’s organization. It is at least conceivable that the stronger 
‘determinants,’ i.e., the particles embodying the rudiments 
of certain qualities, will make more of the food-supply than 
those which are weaker, and that a selective process will 
ensue” (Thomson). This is the conception of germinal 
selection. 
He has also extended the application of the general 
doctrine of natural selection by supplying a great number 
of rfew illustrations. = 
The whole theory of Weismann is so well constructed 
that it is very alluring. Each successive position is worked 
