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ME. 
HOOKER LECTURE, 1917. 123 y 
at random. But the prevalence of parallel development and convergence 
suggests that they are not produced at random. Possibly such progressions 
may be directed by some internal or physiological necessity. They raise, 
however, in my mind very forcibly the question whether or not these 
changes are promoted, or actually determined in their direction, or their 
number, or their quality, in some way by the external conditions. I am 
not aware of any facts which would raise this beyond the level of reasonable 
suggestion, or probability. It is far from being proved; but it is equally far 
from being disproved. It is in fact an open question. Until the contrary 
is proved it would, in my opinion, be wiser to entertain as a working hvpo- 
thesis some such view as that suggested than positively to deny it. The 
impress of external circumstance cannot properly be ruled out in the genesis 
of inheritable characters simply because up to the present time no definite 
case of inheritance of observable characters acquired in the individual life- 
time has been demonstrated. Already evidence is available from the side of 
Zoology which, though it may not yet amount to demonstration, makes the 
negation of inheritance of acquired characters perilous. A single positive 
observation may at a stroke upset the whole negative position. Moreover, 
the prevalence of parallel and convergent characters has made that position 
suspect to many of those who pursue Morphology, whether of the animal or 
of the vegetable kingdom. More especially I have found that this doubt is 
entertained by those who have lived in the atmosphere of experiment and 
observation found in large Botanic Gardens. 
Plants would seem to be particularly favourable subjects for observation 
in testing this question. The early segregation of the germ-cells in the 
animal body was a fact which weighed greatly with Weismann in his 
negation of the inheritance of acquired characters. But in Plants that 
early segregation does not take place. In them the tissues, undifferentiated 
as somatic and germ-cells, are for long exposed to the conditions under 
which vegetation is carried out, before the germ-cells are specialized. When 
this circumstance is given its full weight, such results of comparison as we 
have been discussing seem to me more intelligible if related causally in some 
way with the external conditions under which evolution proceeds, than if that 
be denied. 
There are various ways of approaching the problem, such as the com- 
parative, the experimental, and the line of Mendelian analysis. Those who 
approach the facts by the last should keep an observant eye also on the 
methods and the conclusions of those who use a different method of enquiry. 
More particularly they should watch with sympathetic interest those who 
pursue the morphological avenue of approach to final truth: and especially 
when these observers are by pursuit of that avenue led to conclusions 
divergent from their own. Many Morphologists have found themselves 
unable to accept that general application of the theory of “inhibiting 
LINN. JOURN.—BOTANY, VOL. XLIV, d 
