Everywhere the order of things changes so gradually that man cannot observe the change directly, and the animal tribes in every place preserve their habits for a long time; whence arises the apparent constancy of what we call species--a constancy which has given birth in us to the idea that these races are as old as nature. But the surface of the habitable globe varies in nature, situation, and climate, in every variety of degrees. The naturalist will perceive that just in proportion as the environment is notably changed will the species change their characters. It must always be recognised: (1) That every considerable and constant change in the environment of a race of animals works a real change in their wants. (2) That every change in their wants necessitates new actions to supply them, and consequently new habits. (3) That every new want calling for new actions for its satisfaction affects the animal in one of two ways. Either it has to make more frequent use of some particular member, and this will develop the part and cause it to increase in size; or it must employ new members which will grow in the animal insensibly in response to the inward yearning to satisfy these wants. And this I will presently prove from known facts. How the new wants have been able to attain satisfaction, and how the new habits have been acquired, it will be easy to see if regard be had to the two following laws, which observation has always confirmed. FIRST LAW.--In every animal which has not arrived at the term of its developments, the more frequent and sustained use of any organ strengthens, develops, and enlarges that organ, and gives it a power commensurate with the duration of this employment of it. On the other hand, constant disuse of such organ weakens it by degrees, causes it to deteriorate, and progressively diminishes its faculties, so that in the end it disappears. SECOND LAW.--All qualities naturally acquired by individuals as the result of circumstances to which their race is exposed for a considerable time, or as a consequence of a predominant employment or the disuse of a certain organ, nature preserves to individual offspring; provided that the acquired modifications are common to the two sexes, or, at least, to both parents of the individual offspring. Naturalists have observed that the members of animals are adapted to their use, and thence have concluded hitherto that the formation of the members has led to their appropriate employment. Now, this is an error. For observation plainly shows that, on the contrary, the development of the members has been caused by their need and use; that these have caused them to come into existence where they were wanting. But let us examine the facts which bear upon the effects of employment or disuse of organs resulting from the habits which a race has been compelled to form. _II.--The Penalties of Disuse_