664 The American Naturalist. [August, 
lieved that the nature or direction or particular kind of varia- 
tion in a given case, is determined very largely by the consti- 
tution of the organism, but variation itself, that is, variability, 
proceeds largely from external causes; and the characters 
arising in the lifetime of an individual may become hereditary. 
I must hasten to explain, however, that Darwin clearly recog- 
nized the importance of the union of sexes, or crossing, as a 
cause of variation. 
While Darwin believed that the effects of variability arise 
“ generally from changed conditions acting during successive 
generations," he nevertheless believed that the first increment 
of change—that arising in the first individual of a given 
series—might be directly carried over to the first offspring. 
That is, he believed in the hereditability of acquired or new 
external characters, a fact which is emphasized by his convic- 
tion that certain mutilations, and even the effects of use and 
disuse, may be transmitted. Yet, whilst Darwin accepted the 
doctrine, he believed it much less thoroughly than Lamarck 
did, and it is but an incidental part of his philosophy, while 
it is an essential tenet of Lamarckism. 
Thus far, the heriditability of all important characters had 
not been disputed. In other words, heredity as a general law 
or foree in the organie world, had been assumed. But with 
the refinement of the discussions it became necessary to con- 
ceive of some definite means through which the transmission 
of particular characters or features should operate; and it was 
soon found, also, that no philosophy of evolution can expect 
to explain the phenomena of organic life unless it is connected 
and co-ordinated with some hypothesis of the method of her- 
edity. While, therefore,-a hypothesis of heredity need not 
necessarily be associated with the abstract theory of evolution, 
all such hypotheses which are now before the scientific world 
have for their particular object the explanation of the assumed 
progressive tendency of the forms of life. 
It is incomprehensible that the minute fertilized ovum or 
ovule should reconstruet the essential characters of the two 
individuals from which it proceeds, unless it has in some way 
derived distinct impressions from every part and organ of the 
