54개의 결과 중 1 - 5개
... in which works he upholds the doctrine that species are descended from each
other. He seems to have been chiefly led to this conclusion by the difficulty of
distinguishing species and varieties, — by the almost perfect gradation of the
... Cases of difficulty — Natura non faclt saltum — Organs of small importance —
Organs-irot in all cases absolutely perfect— The law of Unity of Type and of tho
Conditions of Existence embraced by the theory of Natural Selection, .
In many cases we do not know what the aboriginal stock was, and so could not
tell whether or not nearly perfect reversion had ensued. It would be quite .
necessary, in order to prevent the effects of intercrossing, that only a single
With these exceptions (and with that of the perfect fertility of varieties when
crossed, — a subject hereafter to be discussed), domestic races of the same
species differ from each other in the same manner as, only in most cases in a -
The period at which the perfect plumage is acquired varies, as does the state of
the down with which the nestling birds are clothed when hatched. The shape and
size of the eggs vary. The manner of flight differs remarkably; as does in some ...
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