35 
1 877.] Evolution by Expansion . 
I have stated that larvae in their development ascend 
in the scale of organisation. Darwin says that some 
descend. If the faCt of the descent be real, and not only 
apparent, it presents a very formidable difficulty. But, 
as Darwin also remarks, it is very difficult to determine 
what constitutes the higher organism. It certainly is not 
superiority in any particular quality, or greater perfection of 
any one organ. He says that the embryo in course of 
development generally rises in organisation, and that he 
uses that expression though he is aware that it is hardly 
possible to define clearly what is meant by the organisation 
being higher or lower. Considering that development is 
almost, if not quite, universally progressive, and not retrogres- 
sive, — in short, that progress is the established law of Nature, 
— the higher organism may, I think, safely be defined to be 
that which has been produced in the later stage of evolution. 
Characteristics acquired by natural selection alone would 
supersede, permanently and entirely, earlier adaptations, as 
there is no reason deducible from the supposition that the 
formation of species is wholly due to the conditions of 
existence, to account for organisms reverting to ancestral 
types in their embryos or larvse. Peculiarities of structure, 
the result of extraneous influences, when once lost or re- 
placed by others, could not recur either in the embryonic 
state or at maturity without the exciting cause of the same 
extraneous influences. Ancestral type alone is reproduced 
in the embryo, and not the modification of type due to 
natural selection,— the grand leading characters, but not the 
innumerable adaptations. 
Thus the phenomena of embryology, while strongly sup- 
porting the theory of descent with modification, decidedly 
militate against the supposition that all structural changes 
are due to natural selection. 
Darwin, in quoting Mr. Lewes, remarks that the tadpole 
of the common salamander or water-newt “ has gills, and 
passes its existence in the water ; but the Salamandra atm, 
which lives high up among the mountains, brings forth its 
young full-formed. This animal never lives in the water. 
Yet if we open a gravid female, we find tadpoles inside her 
with exquisitely feathered gills ; and when placed in water 
they swim about like the tadpoles of the common water- 
newt. Obviously this aquatic organisation has no reference 
to the future life of the animal, nor has it any adaptation to 
its embryonic condition ; il has solely reference to ancestral 
adaptations , — it repeats a phase in the development of its pro - 
genitors .” 
D3 
