of Edinburgh, Session 1864-65. 273 
these phenomena have suggested to the mind of those who have 
speculated on the origin and development of species. 
There is one idea which has been common to all theories of 
development, and that is the idea that ordinary generation has 
somehow been producing, from time to time, extraordinary effects, 
and that a new species is, in fact, simply an unusual birth. It is 
worthy of observation, that the earlier forms in which the theory 
of development appeared, did suggest something more nearly ap- 
proaching to a law of creation than is contained in the later form 
which that theory has assumed in tlie hands of Mr Darwin. The 
essential idea of the theory of development, in its earlier forms, 
was, that modifications of structure arose somehow by way of 
natural consequence from the outward circumstances or physical 
conditions, which required them, and from the living effort of organ- 
isms sensible in some degree of that requirement. Now, inadequate 
and even grotesque though this idea may be as explaining the 
origin of new species, it cannot be denied, that it makes its appeal 
to a process which, at least to a limited extent, does operate in pro- 
ducing modifications of organic structure. For example, the same 
species of mollusc has often a shell comparatively weak and thin, 
or a shell comparatively robust and strong, according as it lies 
in tranquil or in stormy water. The shell which is much ex- 
posed needs to be stronger than the shell which is less exposed. 
But the mere fact of the need cannot supply the thing needed, unless 
by the adjustment of some machinery for the purpose. How the 
vital forces of the mollusc can thus be made to work to order, 
under a change of external conditions, we do not know. But we 
do know, as a matter of fact, that the shell is thickened and 
strengthened, according as it needs resisting power. This result does 
not appear to arise from any difference in the amount of lime held 
in solution in the water, but upon some power in the secreting 
organs of the animal to appropriate more or less of it, according to 
its own need. The effects of this power are seen where there is no 
difference of condition except difference of exposure. I have seen 
it stated, that they are observable in the shells which lie on the 
different sides of Plymouth breakwater, — the sheltered side and the 
exposed side. The same power of adaptation is seen in many other 
forms. Trees which are most exposed to the blast are the most 
