30 Evolution by Expansion . [January, 
the conditions of existence, which, however, wholly influence 
natural selection. In illustration of this I cannot do better 
than quote Darwin. When writing on the subject of 
morphology, he says — 1 “ This is one of the most interesting 
departments of natural history, and may be said to be its 
very soul. What can be more curious than that the hand 
of a man, formed for grasping, that of a mole for digging, 
the leg of a horse, the paddle of a porpoise, and the wing of 
a bat, should all be constructed on the same pattern, and 
should include similar bones, in the same relative positions ? 
How curious it is, to give a subordinate though striking in- 
stance, that the hind feet of the kangaroo, which are so 
well fitted for bounding over the open plains, — those of the 
climbing, leaf-eating koala equally well fitted for grasping 
the branches of trees, — those of the ground-dwelling, inseCt - 
or root-eating bandicoots, — and those of some other Aus- 
tralian marsupials, — should all be constructed on the same 
extraordinary type, namely, with the bones of the second 
and third digits extremely slender and enveloped within the 
same skin, so that they appear like a single toe furnished 
with two claws. Notwithstanding this similarity of pattern, 
it is obvious that the hind feet of these several animals are 
used for as widely different purposes as it is possible to con- 
ceive. The case is rendered all the more striking by the 
American opossums, which follow nearly the same habits of 
life as some of their Australian relatives, having feet con- 
structed on the ordinary plan. Prof. Flower, from whom 
these statements are taken, remarks in conclusion — ‘ We 
may call this conformity to type, without getting much 
nearer to an explanation of the phenomenon and then he 
adds — ‘ but is it not powerfully suggestive of true relation- 
ship, of inheritance from a common ancestor?’ ” 
It is certainly suggestive of this, and of much more. It 
seems to show very clearly that morphological characters, 
referable to type only, do not vary with the conditions of 
existence. 
A very remarkable instance of adaptation to the conditions 
of existence is exhibited by many species of flat-fish, which 
swim with the median plane horizontal, instead of vertical 
as in other fishes, and which, while maintaining their type, 
have heads actually distorted in a very singular manner to 
suit their habits. 
Were natural selection the sole architect of animal life, it 
would be expected that members fulfilling similar offices 
would be similarly constructed, instead of conforming to the 
inexorable law of unity of type. But we find that any part of 
