We have given in the previous chapter six factors of evolution—viz.: 1. Pressure of the environment. 2. Use and disuse of parts. 3. Natural selection. 4. Sexual selection. 5. Physiological selection. 6. Reason. Let us now compare these as to their grade in the scale of energy and as to the order of their introduction.

The first two or the Lamarckian factors are the lowest in position, the most fundamental and universal, and therefore the first in the order of appearance. They precede all other factors, and were doubtless for a long time the only ones in operation. For, observe, all the selective factors—i.e., those of Darwin and Romanes—are conditioned on reproduction; for the changes produced by these are not in the individual during life, but in the offspring at birth. And not only so, but the operations of these factors are further conditioned on sexual modes of reproduction; for all the non-sexual modes of reproduction—as, for example, by fissure and by budding—are but slight modifications of growth, and the resulting multitude of organisms may be regarded as in some sense only an extension of the first individual. Of course, therefore, the identical characters of the first individual are continued indefinitely, except in so far as they are modified in successive generations by the effect of the environment and by use and disuse—i.e., by the Lamarckian factors.