56 GENERAL PHYSIOLOGY 



to look with scorn upon those attempts of the middle ages, when we 

 realise that from that time even to the most recent period the at- 

 tempts have been continued to produce artificially not man himself, 

 but the simplest forms of living substance. Yet all these attempts 

 resemble the endeavour of a man to put together a complicated 

 clock-work without knowing its essential parts. However simple 

 the problem of the artificial production of living substance appeared 

 to the middle ages, the progress of sober thought and critical in- 

 vestigation has shown constantly how far we are yet removed 

 from a knowledge of the intimate composition of such substance. 

 How is it possible to produce chemically a substance the chemical 

 composition of which is not at all known ? Modern research has 

 been directed, therefore, more and more toward an examination of 

 the composition of living substance. It has penetrated deeply, and 

 continues to penetrate, into the morphological, physical and chemical 

 relations, and the intimate structure of living matter. 



A. THE INDIVIDUALISATION OF LIVING SUBSTANCE 



1. The Cell as an Elementary Organism 



When the organic world inhabiting the surface of the earth is 

 examined, it is found that living substance does not form a single 

 coherent mass, but that it is divided into separate organic indivi- 

 duals. It is not wholly easy to define the conception of the organic 

 individual ; yet many investigators, in recent times particularly 

 Haeckel ('66), have endeavoured to give it a generally valid form. 

 It arose in early times by a process of abstraction from ideas of 

 man and the higher animals, which appear as unitary living beings 

 independent of one another. But, as with all such early concep- 

 tions which spring from a limited circle of experiences and later 

 come to cover a larger circle, the conception of the individual in 

 its original form has become too narrow and requires an extension. 



The original idea upon which the conception of individuality 

 was based, was that of indivisibility. According to this an indi- 

 vidual was a unitary whole, which was incapable of division with- 

 out losing its characteristic properties. So long as none but men, 

 vertebrates and perhaps insects were in mind this definition held 

 good, for a man, a vertebrate or an insect cannot be divided into 

 several independent individuals. But difficulties appear when we 

 descend lower in the animal series or attempt to apply the con- 

 ception to plants. 



In fresh-water ponds and lakes there exists a peculiar repre- 

 sentative of the great group of Cnidaria, the fresh-water polyp 

 Hydra. This small animal, about one centimetre long, with its 

 slender tube-like body bearing several long thread-like tentacles 

 that serve for catching prey (Fig. 2, A\ began to attract the at- 



