Another argument is drawn from the coadaptation of parts. We know that the male sheep, likewise the goat, the stag, and the males of many other animals, have large horns. They are supposed to be useful in fighting with rivals in order to secure as large a number of females as possible. Now these large horns require at the same time a greater development of the bones of the head to hold them, also larger and stronger vertebrae of the neck and back, and larger muscles of these parts to maintain and use them effectively. In other words, there must be coadaptation of all the parts, otherwise these larger horns would be an incumbrance and useless. Now, if we accept the theory of the inheritance of acquired characters, this is all simple. The use of the head in butting against other males exercises all these parts simultaneously, and they develop equally and at the same time. If, however, inheritance has no part in the matter, then we must fall back on variation in the germ-plasm and natural selection for an explanation; but it is difficult or, as Spencer says, impossible to conceive of variation producing large and heavy horns on these animals and at the same time coadaptation of all the other parts to hold and use them. Sometimes coadaptation does not take place, as in the common brook crab, familiar to every country boy. Its foreclaws or fingers are out of all proportion to the rest of the leg, and its awkwardness is well known. The lobster is another case. Even in human beings we have instances of non-coadaptation, as where the head and brain are out of proportion to the size of the body, or the reverse. I need not multiply instances. Now, if acquired characters are transmitted, any system of training which exists for a considerable time must necessarily appear in the structure of the body and in the character. If the training is not in accord with the laws of evolution, it causes the race to deviate from the true line of progress, and by just so much hinder advancement. If, on the other hand, our systems of education conform to correct principles, progress is advanced by them.