We have stated this view in a too extreme form, in order to make it clearer. We now, therefore, proceed to qualify somewhat. Specific types were held, by writers of this school of thought, to be _substantially_ but not absolutely unchangeable. Successive individuals of the same species were admitted to be not exactly alike. Such slight differences were called _varieties_. It was admitted, indeed, that species varied, but it was believed that such variations in any direction were strictly limited in amount. A species may be compared to a right cylinder standing on end. As such a cylinder may be tilted slightly in one direction or another, without overthrowing its equilibrium, the cylinder tending ever to right itself and return to its original position, so a species may be varied slightly in one direction or another without destroying its integrity, the species tending ever to return to its normal or typical form. But as the cylinder, if pushed too far from its normal position, is overthrown, so also a species, if pressed too far in the way of variation from its typical form, is destroyed, but not changed into another species. As cylinders may be more or less rigid, depending upon the breadth of their bases, so also some species are more rigidly set in their typical form, and some are more plastic to influences causing variations, but in all cases there is a limit to the amount of oscillation consistent with integrity. =The New View briefly stated.=--According to Darwin, and all biologists of the present day, species are variable _without limit_, if only the causes of change are constant and slow enough in their operation, and the time long enough. A species must be in harmony with its environment, for this is the condition of its existence. Now, if the environment change, the species must _tend_ to change slowly from generation to generation, so as to readjust its relations in harmony with the changing environment. If the change of environment be slow, the readjustment may be successful, and the species will change gradually into another form, so different that it will be called a different species, especially if the intermediate gradations be destroyed. If the change in the environment be too rapid, many species, especially the more rigid, will be destroyed, while the more plastic may survive by modification. Thus, at every step in the evolution of the organic kingdom, some species have died without issue, while others have saved themselves by changing into new forms in harmony with the new environment. Comparing to a growing tree, some branches overshadowed die, while others push on for light, forming new lateral buds, and dividing as they grow. By continued divergent change species gradually become genera, genera families, etc. Thus, varieties, species, genera, families, orders, classes, etc., are only different degrees of differences formed all in the same way. Varieties are only commencing species, species commencing genera, and so on.