INTRODUCTION 



in fact, also parasites. This may prove eventually to be susceptible 

 of something like demonstration, but in the meantime we must 

 ask where the limit to this assumption that chlorophyll is of 

 parasitic origin is to be placed. 



It cannot be that all chlorophyll even that observed in all uni- 

 cellular plants and animals is to be regarded as " parasitic." And 

 if we are once able to distinguish certain independent unicellular 

 organisms which actually manufacture chlorophyll Avithin them- 

 selves by the activity of their own protoplasm, we shall be able to 

 study the steps of that process and to judge as to whether the 

 protoplasm of the green cells of green plants and of the freshwater 

 sponge and of the green Hydra do or do not form chlorophyll 

 plastids in the same way and in virtue of the same protoplasmic 

 capacity as do minute unicellular algae. 



There is no reason, a priori, for refusing to ascribe to a tissue- 

 cell of a Sponge or a Hydra the same capacity to form a chemical 

 deposit of any kind which a free unicellular organism possesses. 

 Unfortunately this is not a case in which the simple test of observa- 

 tion can be applied, so that the question as to whether the tissue- 

 cell does construct a chlorophyll -corpuscle or does not can be settled 

 by inspection. The intricacies of structure and growth are in this 

 matter such as to render direct observation difficult and illusive. 



Whilst there are, then, exceptional cases in both plants and 

 animals as to the great nutritional distinction between the two 

 series, it is comparatively easy in all excepting the very lowest 

 forms to satisfy ourselves that the departures from the rule are 

 specialised derivatives from the main series. The colourless or 

 greenless plants are descended from green chlorophylligerous 

 ancestors ; mouthless, gutless animals are descended from mouth- 

 bearing, gut-hollow animals. 



When, however, we come to the very lowest unicellular micro- 

 scopic forms of life, there is greater difficulty in assigning some of 

 the minuter organisms to one side or the other, and to some extent 

 our decision in the matter must depend on the theory we may 

 provisionally adopt as to the nature of the earliest living material, 

 which was the common ancestral matrix from which both the 

 Plant series and the Animal series have developed. The real 

 question in regard to such a theory is as to whether we find 

 reason to suppose that the combination of carbon and nitrogen to 

 build up proteid, and so protoplasm, required, in the earliest state 

 of the earth's surface, the action of sunlight and the chlorophyll 

 screen. We must remember that, though these are now necessary 

 for the purpose of raising carbon, and indirectly nitrogen, from the 

 mineral resting state to the high elaboration of the organic mole- 

 cule, yet it is, after all, living protoplasm which effects this marvel 

 with their assistance ; and it seems (though possibly there are some 



