instance, among men, the great differences frequently observable among the children of the same parents, cannot be due to the parents having acquired and transmitted different traits at different periods of their lives, for if among the offspring of dogs or fowls there are great differences due to inborn variations alone, so also must there be among the offspring of men. Do we not all know families, some members of which are bigger and stronger than the others? In them we see a co-adaptive variation of all the structures, and if among men strength were important as a factor in survival, we should by the survival of the strong individuals get an evolution in strength. Do we not all know families, some members of which can run more swiftly than the others? In them we see a co-adaptive variation of all the structures which subserve speed; and if among men speed were important as a factor in survival, we should by the survival of the swiftest get an evolution in speed. Do we not all know families, some of the members of which are taller than the others? In them we see a co-adaptive variation of many structures; and if among men height were important as a factor in survival, we should by the continued survival of the tallest get an evolution in height. It is needless to multiply examples. Nothing can be more certain than that which is denied by Mr. Spencer, viz. that co-operative parts vary together.
But how shall we surmount the theoretical objection made by him, viz. that it is highly improbable that a thousand co-operative parts should vary, not only together, but proportionately as well? We shall surmount it by taking cognizance of that which has been so strangely overlooked by biologists, viz. that the action of Natural Selection has not only developed complex and heterogeneous structures in the higher animals whereby they are brought into greater har-
