256 
POPULAR SCIENCE REVIEW. 
to account for reversion by bud variation, there yet remain 
that larger class of bud variations wherein there is no sus- 
picion of reversion. 
This latter category, so far as we see, can only be ex- 
plained by Mr. Darwin’s assertion that, in “ cases in which the 
organisation has been modified by changed conditions, the 
increased use or disuse of parts or any other cause, the gem- 
mules cast off from the modified units of the body will be 
themselves modified, and, when sufficiently multiplied, will be 
developed into new and changed structures.” 
But before we can, with propriety, avail ourselves of this 
latter explanation, we have to be satisfied that a change of 
conditions has really been in operation. And this is too often 
beyond our ken. The majority of bud variations not dis- 
tinctly referable to reversion, appear suddenly, without any 
obvious change of external condition, we know not why or 
wherefore. Suppose we admit, on the ground of intrinsic 
probability, the operation of changed conditions, even though 
we may have no direct evidence on the point, we have yet to 
explain how and why one particular shoot on one particular 
part of a plant should be acted on in this way, when there is 
no appreciable reason why it should be influenced more than 
the rest. 
There is still another way of explaining the phenomena on 
the gemmule hypothesis, and that is, by supposing changes in 
the number, arrangement, or position of the gemmules ; and 
this supposition, though plausible, is yet based on a number of 
mere assumptions, and, moreover, it leaves the cause of the 
altered condition of the gemmules entirely unexplained. 
To sum up, then, we may say that there is no absolute 
difference between bud variation and seed variation. The 
changes manifest themselves in the same manner and in the 
same organs in the case of buds or seedlings respectively. The 
conditions, so far as we know, that produce variation in the 
one are the same that are effectual in the other. Lastly, apart 
from the different mode of origin, there is no essential diffe- 
rence between a bud formed as the result of fertilisation, i.e ., 
an embryo, and one formed without the direct agency of the 
two sexes, i.e., a bud. 
