D,— ZOOLOGY. 83 



and (lay. That the piobleiu was successl'ully solved \vu know, but as 

 to the mechanism of its solution we have no key. It is at this point 

 again, to use Cliurch's words, that the ' plasma, previously within the 

 connotation of chemical proteid matter, becomes an autotrophic, increas- 

 ingly self-regulated, and so far individualised entity, to which the term 

 " life " is applied.' 



The elementary plasma is thus now fairly launched as an individual 

 living organism, and the great fundamental problems of biology — 

 memory, heredity, variation, adaptation — face us at each step of our 

 further progress. We see in broad outhne the conditions the advancing 

 organism had to meet, we see the means by which those conditions 

 were in fact met, we know that only those individuals survived which 

 were able tx>meet them. Further than this we, the biologists of to-day, 

 have not advanced. The younger generation will pursue the quest, 

 and, with patient effort, much that now lies hidden will grow clear. 



The differentiation of tlie growing particles of plasma into definite 

 layers, which followed, seems natural; first the external layer, in mole- 

 cular contact with the surrounding water, from which it receives sub- 

 stances from outside in the form of ions, and to which it itself gives 

 off ions ; beneath this the autotrophic layer to which light penetrates, 

 and in which, under the influence of the light, new organic substance 

 is built up; in the centre a layer to which light no longer penetrates. 

 This central region, the nucleus, depends entirely on the peripheral 

 layers for its own nutrition, and becomes itself concerned only with 

 katabolic processes, those processes of the organism which depend upon 

 the breaking down, and not the building up, of organic substance. 



At an early stage in the development of the individual organism 

 the spherical shape, which the organic plasma was compelled to assume 

 under the influence of surface-tension, underwent an important modifica- 

 tion, the effect of which has impressed itself upon all later developments. 

 A spherical organism floating in the water and growing under the direct 

 influence of light would obviously grow more rapidly on the upper side, 

 where the light first strikes it, than it would on the lower side away 

 from the light. There followed, therefore, an elongation of the sphere 

 in the vertical direction, and the definite establishment of an anterior 

 end, the upper end which lay towards the light and at which the most 

 vigorous growth took place. In this way there was established a 

 definite polarity, which has persisted in all higher organisms, a distinc- 

 tion between an anterior and a posterior end. With the concentration 

 oi organic substance which took the form of nucleus and reserve food 

 supply, the specific gravity of the plasma would become greater than 

 that of the surrounding water and the organism would tend to sink. 

 The necessity, therefore, arose for some means of keeping it near the 

 surface, that it might continue to grow under the influence of light. 

 The response to this need, however it was attained, came in the de- 

 velopment of an anterior flagellum. This we may regard as an elonga- 

 tion in the direction of the light of a contractile portion of the external 

 layer, moving rhythmically, which by its movement counteracted the 

 action of gravity, and acting as a tractor drew the primitive flagellate 

 ujjwards towards the surface layers, into a position where further growth 



