Now, _non-sexual_ reproduction is _absolute true breeding_. The law of like producing like is absolute. Heredity is all-powerful, and tendency to variation is _nil_. These modes of reproduction are in fact but a modification of growth and an extension of the individual. Evolution-changes in animals produced in this way only must be very slow, since the most powerful factor of evolution, viz., natural selection among divergent varieties of offspring, would be wanting. In the earliest times, therefore, before sex was yet declared, we may imagine that physical environment was the great and only factor of change. Sexual reproduction introduces the new element of variation of offspring from which Nature makes her selections; and this element of variation is apparently the result of the union of _diverse_ individuals, and the funding of these differences in a common offspring, and thus a double inheritance of individual characteristics from the parents and a multiple inheritance of the same from the ancestry. See, then, with this end in view, the pains Nature has taken to make the difference between the uniting individuals and the diversity of inheritance by the offspring as great as possible, and yet the gradual way in which she has accomplished it. As already said, the lowest form of reproduction is that by _fission_. Next comes budding in _any_ part indifferently. Next comes the relegation of the budding function to a particular part. This is the first appearance of a reproductive _organ_. Next comes the placing of this organ, for greater safety, within. Thus far all is non-sexual reproduction--all a modification of growth--an extension of the individual, like the propagation of plants by cuttings and by buds. Then comes sexual reproduction in its lowest forms.