176 NATURAL SCIENCE [September 
in adaptation to the aérial medium. Numerous other illustrations 
could be given.t ) 
I think it must be from not being aware of the abundance of 
evidence of this sort, that the idea arose that all the offspring did 
not acquire the same characters when the external conditions were 
changed. 
Dr Wallace doubts it because, he says, “the argument is, that 
the same causes will always produce the same or closely similar 
results. But this is only true when the same causes act upon 
identical materials and under identical conditions”? Dr 
Wailace is mistaken in supposing that nature pays any attention 
to “individual differences” which occur between any number of 
plants of the same kind. It is not a question of argument, but 
of facts. These differences are of no moment whatever when 
self-adaptation is required to take place. The external influences 
cause all the individuals to change alike in the same direction, and 
utterly ignore the various dimensions among the “individual differ- 
ences” described above. The result is that the same facies is ac- 
quired by all the individuals, though a new set of individual differences 
may readily be found among the individuals of the new variety. 
Secondly, besides doubting the occurrence of the same definite 
variations in the whole of the offspring subjected to new but 
similar external conditions, Dr Wallace adds :—“ It must do more 
than this, for it must produce a variation so exceptionally stable 
that it constantly recurs in all the offspring of successive genera- 
tions, even though those offsprings are subjected to considerable 
change of conditions.” * 
But the stability of a species, I repeat, is a purely relative 
matter and depends upon time. Some plants are very plastic, 
others are not so, some variations may become very (but never 
absolutely) rigidly fixed, while others may refuse to be reproduced 
by seed with any certainty at all. Not only is this true when the 
plant is propagated by seed but it is also true for vegetative multi- 
plication. Tulips, &c., introduced from the East, though they have 
presumably been constant in form for unknown ages, yet often 
become unrecognisable in three years though propagated by bulbils 
only ; apple trees, though propagated by grafts alone have given rise 
to numerous varieties; even different kinds of apples raised on 
stocks, but grown in the same States of N. America, respectively, 
often bear fruit of approximately the same form. Qn the other 
hand the Jerusalem artichoke, asparagus, sea-kale and celery offer 
The reader is again referred to ‘‘The Origin of Plant Structures” for further 
ae cit., p. 488. 3 Loc. cit., p. 489. 
4 «Bud Variation and’ Evolution,” Natural Science, vol. vii., p. 103. An essay in 
Mr Bailey’s work ‘‘ The Survival of the Unlike,” 1896, 
