There is no making and wearing out of dies, and making of new ones; the whole process is a natural one--the whole series is genetically connected. In a perfect classification varieties, species, genera, families, orders, classes, etc., are only different _degrees of blood-kinship_. So much may be regarded as certain, and out of the field of discussion among biologists of the present day. It is only in defining this process more accurately, and especially in the _theory of the causes_ or _factors_ of evolution, that there are still difference and discussion. The most probable view on this subject we now proceed to give. =Factors of Evolution.=--The causes of change or adaptive modification, or the factors of evolution, are at least _four_ well known, and probably many more still unknown: 1. The physical environment--heat and cold, dryness and moisture--affects function of organs, and function affects structure, and both changed function and changed structure are inherited by offspring, and so increased from generation to generation, becoming greater without limit. 2. Increased _use_ or _disuse_ of organs enforced or permitted by change in the environment, physical or organic, or both, induces change in form, size, and structure of the organs; and this change is inherited by the offspring, and so from generation to generation small differences are integrated until they become great without limit. These two factors were recognized by Lamarck. 3. “Natural selection,” or “survival of the fittest,” among divergent varieties of offspring. This is the distinctive Darwinian factor. In the two preceding factors the change is during the _individual lifetime_, and reproduction is supposed to transmit it unchanged to the offspring. In this factor, on the contrary, the form and structure are supposed to remain unchanged during the individual life, but for some unknown cause there are slight variations in different directions (divergent) in the offspring from the same parents. Now, when we remember that by reproduction the number of individuals tends to increase by geometrical progression, and that in each generation only a very few (on an average only two from all the offspring of one pair) can survive, it is evident that among these divergent varieties those will most likely survive which are most in harmony with the external environment, and which possess the most efficient organs of defense or of escape, or for food-taking. The surviving offspring, therefore, will be on the average better in these respects than their parents. It matters not how little better, for the integration of even infinitesimal improvements from generation to generation will eventually produce any required amount of change. 4. To the above Darwin has added also “_sexual selection_.” In _natural_ selection there is struggle of _all_ for _food_, or _means of living_. In sexual selection there is a struggle among the _males_