Look at the race and dray-horse, or at the greyhound and mastiff.

Their whole frames, and even their mental characteristics, have been modified; but if we could trace each step in the history of their transformation--and the latter steps can be traced--we should not see great and simultaneous changes, but first one part and then another slightly modified and improved.

Even when selection has been applied by man to some one character alone--of which our cultivated plants offer the best instances--it will invariably be found that although this one part, whether it be the flower, fruit, or leaves, has been greatly changed, almost all the other parts have been slightly modified.

This may be attributed partly to the principle of correlated growth, and partly to so-called spontaneous variation.
A much more serious objection has been urged by Bronn, and recently by Broca, namely, that many characters appear to be of no service whatever to their possessors, and therefore cannot have been influenced through natural selection.

Bronn adduces the length of the ears and tails in the different species of hares and mice--the complex folds of enamel in the teeth of many animals, and a multitude of analogous cases.