In conclusion, let me again impress upon the reader that all the doubt and discussion, above described, as to the factors of evolution, is entirely aside from the truth of evolution itself, concerning which there is no difference of opinion among thinkers.
THE GRADES OF THE FACTORS OF EVOLUTION AND THE ORDER OF THEIR APPEARANCE.
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. In sexual generation, on the contrary, the characters of two diverse individuals are funded in a common offspring; and the same continuing through successive generations, it is evident that the inheritance in each individual offspring is infinitely multiple. Now, the _tendency to variation_ in offspring _is in proportion to the multiplicity of the inheritance_: for among the infinite number of slightly differing characters, as it were, offered for inheritance in each generation, some individuals will inherit more of one and some more of another character. In a word, sexual reproduction by multiple inheritance _tends to variation of offspring, and thus furnishes material for natural selection_.[16]