one hundred and thirtieth generation members were removed at different stages, and were observed to conjugate successfully with unrelated forms.
"When the family began to draw near its end even removal to fresh conditions was without effect. About the one hundred and eightieth generation, the strange sight was seen of individuals of the same family attempting to unite with one another. The results were however nil, and the conjugates did not even recover from the effects of their forlorn hope.
"Without the normal sexual union then, the family becomes senile. Powers of nutrition, division, and conjugation with unrelated forms comes to a standstill. This senile degeneration is very interesting. The first symptom is decrease in size, which may go on till the individuals may not measure over a quarter of their normal proportions. Various internal structures then degenerate until at last we see formless abortions incapable of living and reproducing themselves."—Thompson, Elements of Zoology.Among these low forms conjugation is evidently the same thing as sexual union among the higher animals, to which, and to the vertebrates in particular, popular observation is almost exclusively directed; and as sexual union among the vertebrates invariably precedes the birth of a new individual, it is naturally supposed that it is the cause of the genesis of a new individual; but this is certainly not so, for various invertebrate species are able to multiply asexually. Some organisms comparatively high in the scale are able to reproduce imperfectly without sexual union; for instance, among bees the offspring of asexual reproduction are drones only, not workers and queens, who are essential to the continuance of the race. Others again, such as aphides, are able to multiply asexually perfectly for many generations before sexual reproduction occurs; and lastly, Weismann has never been able to discover sexual union
