464 Proceedings of the Royal Physical Society. 



past, together with the degree of specialisation to which they 

 attained, and also some well-known, though at present 

 unexplained, facts in zoology and physiology, seem indeed to 

 strongly suggest that there is a time of life for the species 

 or group which is connected with the rapidity with which 

 specialisation takes place. There is an increasing opinion 

 that no form of life can undergo repeated fission, budding, 

 or self-fertilisation for more than a certain number of genera- 

 tions, without bringing about its own extermination. The 

 experiments of Maupas have proved this to be the case at 

 any rate as regards the fission of some of the ciliate Protozoa, 

 and in the higher forms of life there seems to be a necessity 

 for a sexual generation sooner or later to occur to prevent a 

 dying out of the stock. As we proceed from forms of less 

 specialisation to forms of greater specialisation, this cross 

 becomes apparently an increasing necessity, and gradually 

 fission, budding, and all forms of asexual reproduction are 

 lost, and at the same time the powers of architectural repeti- 

 tion and also of reproduction of lost parts becomes less and 

 less evident. Amongst plants this loss of a generalised 

 power is not so obvious, owing to the line of increasing 

 anabolism along which they have specialised, but the 

 increasing necessity for cross fertilisation is quite apparent. 



A cross between individuals not of the same stock, in all 

 cases brings about a greatly increased vitality, or what is 

 usually termed rejuvenescence. In some cases resting-spores 

 are produced, with great powers of resisting death from 

 desiccation or other injuries. In other cases it is the start- 

 ing-point of a very vigorous multiplication, and in all the 

 higher forms of life, of the marvellously rapidly-produced 

 growth and specialisation of embryonic existence. Amongst 

 forms of high specialisation, the blending of individuals of 

 near ancestry seems to have a very detrimental result, with 

 very frequently a tendency to increasing sterility, as well 

 as a general weakness of the individual in relation to its 

 environment. This point, I believe, has been disputed by 

 many zoologists, who hold that there are no ill-effects pro- 

 duced from continued in-breeding ; but it is asserted by 

 Darwin to be the general belief amongst breeders " that 



