368 THE INDUCTIONS OF BIOLOGY. 



separate families : the arrangements and apportionments of 

 duties in each are internally determined. Mark the fact 

 which here chiefly concerns us. This formation of a hetero- 

 geneous aggregate with its variously adapted parts, which 

 while influenced by the whole are mainly self-formed, goes 

 on among units of essentially the same natures, inherited 

 from units who belonged to similar societies. And now, 

 carrying this conception with us, we may dimly perceive 

 how, in a developing embryo, there may take place the form- 

 ation, first of the great divisions the primary layers then 

 of the outlines of systems, then of component organs, and so 

 on continually' with the minor structures contained in major 

 structures; and how each of these progressively smaller 

 divisions develops its own organization, irrespective of the 

 changes going on throughout the rest of the embryo. So 

 that though all parts are composed of physiological units of 

 the same nature, yet everywhere, in virtue of local conditions 

 and the influence of its neighbours, each unit joins in forming 

 the particular structure appropriate to the place. Thus con- 

 ceiving the matter, we may in a vague way understand the 

 strange facts of autogenous development disclosed by the 

 above named experiments. 



97/. " But how immeasurably complex must be the 

 physiological units which can behave thus ! " will be remarked 

 by the reader. " To be able to play all parts, alike as mem- 

 bers of the whole and as members of this or that organ, they 

 must have an unimaginable variety of potentialities in their 

 natures. Each must, indeed, be almost a microcosm within 

 a microcosm." 



Doubtless this is true. Still we have a consensus of proofs 

 that the component units of organisms have constitutions of 

 extremely involved kinds. Contemplate the facts and their 

 implications. (1) Here is some large division of the animal 

 kingdom say the Vertebrata. The component units of all 

 its members have certain fundamental traits in common: all 



