36 PROBLEMS OF EVOLUTION 



the Metazoa the specialisation of the somatic cells is a great 

 advantage, while heredity is maintained by the only slightly 

 varying reproductive cells. And we can imagine Natural Selec- 

 tion singling out for survival those organisms in which these 

 two opposite tendencies were developed simultaneously ; the 

 conservative cells handing on the characters of the race; the 

 progressive and less stable cells changing in the course of the 

 ontogeny, so as to meet the most varied demands, and then 

 a wonderful fact ! when once they have gained the requisite 

 qualities, becoming conservative, and maintaining by non-special- 

 ising fission the specialities they have developed. In the one- 

 celled animals, as I have shown, there must be the same 

 tendencies. But the tendency to specialise must soon be fatal. 

 An infusorian's one cell must be a Jack of all Trades, or he 

 will be eliminated. When, after multitudes of fissions, there- 

 fore, the infusorian grows senile, I believe it is because of 

 failure to divide into similar halves that is, to specialisation. 

 There is in fact a failure of heredity : some characters are over- 

 represented in one half and under-represented in the other. 

 When fission has been repeated hundreds of times it is far from 

 wonderful if in every individual thus produced some essential 

 character is lacking. This becomes all the more probable when 

 we remember that the life histories of the Protozoa and the 

 Metazoa seem to follow parallel lines, a point which, as I have 

 already mentioned, 1 has been well brought out by Mr Archdall 

 Reid. 2 If the cells produced by an amoeba's fissions held 

 together there would be formed a Metazoon : the period of 

 fission in the amoeba corresponds to the life of the soma of 

 the Metazoa, which life begins with the formation of highly 

 specialised cells. In the Metazoa amphimixis is beyond doubt 

 a return to the non-specialising principle. In the infusorian, 

 then, also, may we not regard conjugation in the same light ? 

 Conjugation is the natural corrective of the inevitable tendency 

 of cells to divide unequally. But in the higher animals this 

 specialisation in the soma is an enormous gain. On the other 

 hand, very slight variations are required in the cells set apart 



1 See p. I 2. 2 Present Evolution of Man. 



