'PHYSIOLOGICAL UNITS' 93 



Haeckel (plastidules) and of Verworii (biogens) — the existence of 

 which was postulated by them at later periods. 

 ! « After a careful consideration of all the facts known at the time, 

 what may be termed a ' Law of Heredity ' was formulated by H. 

 Spencer, in which he says': — " Bringing the question to its ultimate 

 and simplest form, we may say that, as on the one hand physiologi- 

 cal units will, because of their special polarities, build themselves 

 into an organism of a special structure ; so, on the other hand, if 

 the structure of this organism is modified by modified function, it 

 will impress some corresponding modification on the structure and 

 polarities of its units. The units and the aggregate must act and 

 react on each other. The forces exercised by each unit on the 

 aggregate and by the aggregate on each unit must ever tend 

 towards a balance. If nothing prevents, the units will mould the 

 aggregate into a form in equilibrium with their pre-existing 

 polarities. If, contrariwise, the aggregate is made by incident 

 actions to take a new form, its forces must tend to remould the 

 units into harmony with this new form. And to say that the 

 physiological units are in any degree so remoulded as to bring their 

 polar forces towards equilibrium with the forces of the modified 

 aggregate, is to say that when sepai-ated in the shape of reproduc- 

 tive centres, the units will tend to build themselves up into an 

 aggregate modified in the same direction. 



Among simple organisms, as we have seen, almost any part of 

 the substance which separates, or is separated, from one of them is 

 capable of developing into a similar simple organism. But as 

 organisms grow more and more complex in their structure, so we 

 find that a difference arises in the reproductive powers of different 

 tissues — till at last the capacity to I'eproduce the entire organism 

 (eitlier without fertilisation or only after this has occurred) becomes 

 in animals restricted to the morphological units which are produced 

 in special organs.^ How much this restriction of the reproductive 

 function is due to a general specialisation is obvious from the fact 

 that it is most marked where complexity of organisation attains 

 its maximum. Complexity of structure necessarily carries with it 



■ " Principles of Biology,'' vol. i. § 84. 



» The necessity for the fertilisation of some of these reproductive elements, 

 and the evolution of sexual differences among the animals and plants in 

 which this necessity obtains, is merely a superadded complexity — a difference of 

 degree and not of kind. The fundamental phenomena of reproduction are 

 essentially similar in sexual and sexless organisms. (See Spencer's " Principles 

 of Biology," vol. i. § 77. 



