400 HISTORY OF THEORIES 



argument which seems useful to-day, we give a brief summary 

 (see Principles of Biology (ist ed.), vol. i. p. 181 et seq.). 



In the growth of an embryo from apparent simplicity to 

 obvious complexity, in the regeneration of lost parts, in the 

 regrowth of a whole by a part, the living substance arranges 

 itself in definite form as some not-living substances do when 

 crystallising out of a solution. In restating the fact, Spencer 

 supposes that certain units composing the living substance possess 

 " polarity," like the chemical units in crystallisation, meaning 

 by " polarity " the unexplained power of definite arrangement. 

 The units cannot be the chemical molecules of albumen and the 

 like, for these do not show the particular kind of differentiation 

 seen in growth ; nor can the units be the cells, for the differen- 

 tiation in question may be seen within the limits of a single cell. 



" There seems no alternative but to suppose that the chemical 

 units combine into units immensely more complex than them- 

 selves, complex as they are ; and that in each organism, the 

 physiological units produced by this further compounding of 

 highly compound atoms have a more or less distinctive character. 

 We must conclude that, in each case, some slight difference of 

 composition in these units, leading to some slight difference in 

 their mutual play of forces, produces a difference in the form 

 which the aggregate of them assumes." 



After the judicious sentences quoted on page 398, Spencer goes 

 on to say : " The applicability of any method of interpretation 

 to two different but allied classes of facts is evidence of its truth. 

 The power which organisms display of reproducing lost parts, 

 we saw to be inexplicable except on the assumption that the 

 units of which any organism is built have an innate tendency 

 to arrange themselves into the shape of that organism. We 

 inferred that these units must be the possessors of special polari- 

 ties, resulting from their special structures ; and that by the 

 mutual play of their polarities they are compelled to take the 

 form of the species to which they belong." This is illustrated 



