222 THE PHILOSOPHY OF BIOLOGY 



work, as well as the analogies which they suggested 

 between organic and inorganic phenomena, compelled 

 the attention of biologists. The essential parts of 

 Weismann's hypothesis, as it was first presented to 

 the world, are as follows : very early in the evolution 

 of living from non-living matter many kinds of life- 

 substance came into existence. These were chemical 

 compounds of great complexity, able to accumulate 

 and expend energy, and capable of indefinite growth 

 and reproduction. They were able to exist in an 

 environment which was hostile to them and which 

 tended always to their dissolution, and which was 

 able to modify their nature and their manner of 

 reacting, though it could not destroy them. These 

 elementary life-substances were very different from 

 those which we know in the world of to-day. They 

 were naked protoplasmic aggregates, undifferentiated 

 into cellular or nuclear plasmata, much less into 

 somatic and germinal tissues. All of their parts were 

 similar, or rather their substance was homogeneous. 

 But even with the evolution of the unicellular organism 

 a profound change was initiated, for henceforth one 

 part of the living entity, the nucleus, became charged 

 with the function of reproduction, although it still 

 continued to exercise general control over the func- 

 tions of the extra-nuclear part of the cell. When the 

 multi-cellular plant and animal became evolved, the 

 heterogeneity of the parts of the organism became 

 greater still. All the cells of the metazoan animal 

 do indeed contain nuclei, but these structures are 

 only the functional centres of the cells : some of the 

 latter are sensory, others motor, others assimilatory, 

 others excretory, and so on. Only in the nuclei which 

 form the essential parts of the reproductive organs does 

 the reproductive function persist in all its entire 



