Again, in the case of a bird, in virtue of its more complex organization, there is command over a much larger area of environment. It can take precautions such as the _Medusa_ could not; it has increased facilities for securing food; its adjustments all round are more complex; and therefore it ought to be able to maintain its Life for a longer period. There is still a large area, however, over which it has no control. Its power of internal change is not complete enough to afford it perfect correspondence with all external changes, and its tenure of Life is to that extent insecure. Its correspondence, moreover, is limited even with regard to those external conditions with which it has been partially established. Thus a bird in ordinary circumstances has no difficulty in adapting itself to changes of temperature, but if these are varied beyond the point at which its capacity of adjustment begins to fail--for example, during an extreme winter--the organism being unable to meet the condition must perish. The human organism, on the other hand, can respond to this external condition, as well as to countless other vicissitudes under which lower forms would inevitably succumb. Man's adjustments are to the largest known area of Environment, and hence he ought to be able furthest to prolong his Life. It becomes evident, then, that as we ascend in the scale of Life we rise also in the scale of longevity. The lowest organisms are, as a rule, short-lived, and the rate of mortality diminishes more or less regularly as we ascend in the animal scale. So extraordinary indeed is the mortality among lowly-organized forms that in most cases a compensation is actually provided, nature endowing them with a marvelously increased fertility in order to guard against absolute extinction. Almost all lower forms are furnished not only with great reproductive powers, but with different methods of propagation, by which, in various circumstances, and in an incredibly short time, the species can be indefinitely multiplied. Ehrenberg found that by the repeated subdivisions of a single _Paramecium_, no fewer than 268,000,000 similar organisms might be produced in one month. This power steadily decreases as we rise higher in the scale, until forms are reached in which one, two, or at most three, come into being at a birth. It decreases, however, because it is no longer needed. These forms have a much longer lease of Life. And it may be taken as a rule, although it has exceptions, that complexity in animal organisms is always associated with longevity.