GREAT STEPS IN ORGANIC EVOLUTION 863 



"machinery" — the more difficult is it for rejuvenescence to keep 

 pace with senescence, especially as regards hard- worked organs. 

 In a higher animal these hard-worked organs are especially the liver, 

 the kidneys, the heart, and the brain — to put them in their probable 

 average order of overwork. Hence the frequency of breakdown in 

 these organs ! 



Natural Death was the price paid for a body, or for one much 

 worth having ; and the Protozoa are largely free from this limitation 

 of all other life, because the whole organism is so relatively simple 

 and so readily amenable to recuperation. Wear and tear are con- 

 tinually made good. Apart from their recurrent "depression" states, 

 the Protozoa are able, in their normal environment, to sustain 

 with persistent success the vital equation between waste and repair. 



The second reason for the relative immunity of the Protozoa 

 from Natural Death is the simplicity of their modes of multiplication. 

 Some of the sea- worms burst in liberating the germ-cells; many 

 insects, such as butterflies and Majrflies, die immediately after 

 reproduction; even strong muscular animals, such as sea-lampreys 

 and common eels, do not seem to recover, in either sex, from the 

 exhaustion of producing and liberating the sex-cells. But it is 

 otherwise with the Protozoa. For a Protozoon divides into two or 

 into many in a relatively simple way which cannot be called physio- 

 logically expensive. The parent organism is directly continued in its 

 progeny, and we can hardly speak of death when there is nothing 

 left to bury. 



A multicellular body had several advantages, such as greater 

 possibility of division of labour, greater possibility of attaining a 

 certain momentum of size and vital capital ; and it was the beginning 

 of the important distinction between somatic cells, forming the body, 

 and germ-cells, continuing the race. This is clearly seen in the 

 Volvox colony, where, in a variety of gradations, there is a setting 

 apart of specialised germ-cells within the hollow ball. But what we 

 have been indicating from the evolutionist point of view is that this 

 great step of body-making was associated with the first appearance 

 of Natural Death, which, while probably inevitable, was probably 

 justified by lessening the risk of multiplying when senescence was 

 dominant. 



THE ORIGIN OF SEX.— We have already referred to the grada- 

 tions among the Protozoa which lead to the fusion of two dimorphic 

 reproductive units, one relatively more anabolic, the other relatively 

 more katabolic. In the Bell Animalcule (VorticeUa) a small free- 

 swimming unit, which might be called male, penetrates into a fixed 

 individual of normal size, which might be called female (Fig. 57). 



It was doubtless by colonies of unicellulars, or by some simple 

 forms of Metazoa or Metaphyta, that another step was taken. 



