34) ORGANIC EVOLUTION— PHYSICAL 



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 at- 

 tempting 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 con- 

 jugation 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 com- 

 paratively high in the scale are able to reproduce im- 

 perfectly 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 gener- 

 ations before sexiial reproduction occurs; and lastly, 

 Weismann has never been able to discover sexual union 



