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get transferred from one host to another. We thus find that in respect to reproduction and life-cycle, parasites are usually much more elaborate than tree-living animals; for instance, the dog tapeworm can be transmitted to a dog only by entering the body of an animal like a rabbit, there going through a special cycle of life, and then being eaten again by a dog.

We usually say that parasites are degenerate, because we note their striking loss of organs and faculties, but they are only particular examples of specialization, with, as usual, elaboration and improvement in one direction and loss in others. The whale, in gaining blubber and tail fin, has lost hair and hind limb; the horse, in improving its middle digit to a hoof, has lost the other four digits on each foot.

But besides such one-sided specialization, there are examples of evolutionary improvement which are all-round, or balanced, and do not deprive their possessors of their precious plasticity. For instance, the change from cold-bloodedness to warm-bloodedness in vertebrates was such a change. In becoming warm-blooded, the bird or the mammal lost nothing which their reptilian ancestors possessed; they merely acquired a new and valuable piece of vital machinery, which enables them to be much more independent of the temperature of the outer world than they were before. In the same way, the reproductive methods of reptiles and birds represent a pure gain when compared with those of their fish-like and amphibian ancestors. The evolution of the protective membrane or amnion, which makes a water cushion round the embryo, and the other embryonic membrane or allantois, which enables the embryo to breathe within the egg-shell, made it possible for reptiles to be independent of water for their breeding, and so helped to open up to them vast tracts of the earth’s surface which