The goal of the breeder is some new form which it pleases him to produce, either for its use, or its beauty, or for its mere singularity. The goal of nature, at least the apparent and immediate goal, is the adaptation of each species to the circumstances of its life. And the first thing that strikes the investigator is the way, often indeed not perfect, but usually most impressive in its apparent thoughtfulness and care, in which the organs of plants and animals are fashioned to secure the most favourable results. But all this is the result of development. The whale is a creature excellently adapted for its present mode of life, but we know that it was once a furry land animal with four legs; the legs are all there still, in modified or rudimentary form, and the fur appears at a certain stage of embryonic development. When we ask, How did this extraordinary transformation come about? what we really mean is, How did the determinants composing the chromatin in the reproductive cells of the original land animal so come to alter as to produce the characteristics of the whale? For new species can only be evolved by means of structural modifications _capable of being transmitted by inheritance_; and nothing can be inherited except through the action of the determinants. A modification which does not affect the reproductive cells has no significance in the evolution of species. To this question Darwinism has given us our choice of two answers, which may be termed respectively the Darwin-Lamarck and the Darwin-Weismann theories. Lamarck explained the origin of species by the accumulated effect of the inheritance, through many generations, of modifications acquired by the exercise, or the disuse, of the modified organs. Observing that living protoplasm responds to demands upon it (thus, for instance, a muscle when systematically exercised attracts more nourishment from the blood and grows stronger, and callosities form to protect the skin of the hands of a manual worker), he assumed that modifications so acquired might be transmitted by inheritance. Each new generation, then, would start with a slightly better equipment in this particular respect than the former one had when it started; and so, by slow degrees, a new organ, or one markedly differing from the original form, might be built up. The world, since protoplasmic life first appeared upon it, has gone through many changes, and has always presented a vast variety of climatic and other conditions, calling for the most varied types of organic structure. As animal life gradually spread over the earth and sea, the effort to cope with the different conditions it met with would gradually, by the combined action of exercise, of disuse, and of heredity, produce multitudes of different types; and these are what we know as families, orders, genera, and species. When a species is fairly well adapted to its surroundings and way of life it may go on indefinitely without change.