338 



polarity. Hence in an aggregate of colloidal molecules, indi- 

 vidual molecules would not tend to any special relative dis- 

 position ; in other words, could not crystallise, and con- 

 sequently masses of different colloidal substances exhibit, on 

 the whole, not very diverse external characters. 



But colloidal molecules having polarities so variable, what 

 relation can there exist between them and the vast variety of 

 the shapes of living things ? In the gelatinous state charac- 

 teristic of colloids these polarities are hardly sufficient, even 

 to differentiate their external appearances from one another. 

 It is quite evident then that they cannot possess the property 

 of arranging themselves into the special structures of the 

 organisms to Avhich they belong, since the infinite variety 

 which these present would be inexplicable on such a sup- 

 position. We must agree with Mr. Spencer in conceiving it 

 possessed by certain intermediate or physiological units com- 

 posed of chemical units or molecules, but infinitely more 

 complex, and possessing a more or less distinctive character, 

 which ultimately produces a difference in the forms assumed 

 by the aggregate. This helps us to understand the repetition 

 in the offspring of the peculiarities of the parents if sperm 

 cells and germ cells are essentially nothing more than vehicles 

 of small groups of " physiological units in a fit state for obey- 

 ing their .proclivity towards the structural arrangement of 

 the species they belong to.'^ If this is true of the higher 

 organisms, it must be equally true of the lower ; and as it is 

 impossible to say where a line could be drawn, it probably 

 holds of all living beings, even of those lowest, so destitute of 

 structure as to have no claim to the title of organism at all. 

 Of course it may bo objected that physiological units are 

 mere figments of the imagination, but as much might be said 

 against chemical units. In either case the use of symbolic 

 terms is forced upon us by the analysis of our scientific ideas. 

 In both cases they may be purely arbitrary conceptions ; but 

 if we use one, it is impossible to object to the use of the other. 

 While, therefore, molecules having definite polarities aggre- 

 gate into definite crystalline forms, molecules Avhose polarities 

 are feeble will aggregate into amorphous colloidal masses. 

 The shapes of living bodies are related to organic units more 

 complex than the molecules of colloids. Between a solution 

 of colloidal matter and a bacterium there is a distinct step 

 in integration, which does not exist in the case of the forma- 

 tion of a crystal at all, and this, therefore, is not really analo- 

 gous to the formation of a bacterium from such a solution, 

 supposing it to take place in the way which is described. A 

 ' 'Principles of Biology,' vol. i, p. 254. 



