332 THE EVOLUTION THEORY 



unicellular organisms were not directly contrary to the clear facts of 

 the case ; but this is just what it is. 



We are acquainted with such contradictory facts, not perhaps 

 among the unicellulars themselves, where it is difficult to procure 

 direct proof, but in regard to the germ-cells of many-celled organisms 

 which correspond to unicellular organisms. We know that under 

 certain circumstances the ovum is capable of persisting by itself — in 

 cases of parthenogenesis — how then can we conclude that amphimixis 

 is in the case of Metazoan germ-cells the cause of their capacity for 

 development? We can only conclude, it seems to me, that their 

 power of developing is usually bound up with the occurrence of 

 amphimixis. So we may conclude in regard to the unicellulars that 

 their unlimited power of multiplication is bound up with the occur- 

 rence of amphimixis, but not that amphimixis is the cause of this 

 power, or that it implies a rejuvenescence of life. If unicellular 

 organisms could have been made immortal through amphimixis, then 

 what I maintain would be proved — that they possess potential im- 

 mortality ; but if they did not possess it, no artifice in the world could 

 give it to them ; amphimixis could be at most only the condition with 

 the fulfilment of which the realization of their immortality was 

 bound up. 



One may ask, How then can amphimixis be a condition of their 

 survival ? why should Infusorians which have not conjugated at the 

 proper time be doomed to extinction ? And from the standpoint of our 

 present knowledge I am as little able to give a precise answer as my 

 opponents. But I can give one in relation to the amphimixis of 

 multicellular organisms, for in regard to these Ave know that each 

 of the germ-cells — male and female — uniting in fertilization, is of 

 itself incapable of development and doomed to perish, the sperm-cell 

 because it is too small in mass to be able to develop the whole 

 organism, and the ovum because, in order to become capable of being 

 fertilized, it must undergo certain changes which make it incapable of 

 independent development. We have seen that after the two maturing 

 divisions in the egg-cell have been accomplished the ovum no longer 

 contains a mechanism of division, as the centrosphere breaks up after 

 the second division ; embryonic development can therefore only begin 

 when a new centrosphere has been introduced into the ovum, and 

 this is normally brought about by fertilization, that is, by the 

 entrance of the sperm-cell, whose nucleus is accompanied by a centro- 

 sphere. 



Thus amphimixis is seen to be really a condition of development. 

 But we now know that the ovum can emancipate itself from this con- 



