FROM ONE CELL TO MANY CELLS 65 



tomed to building materials whose nature depends on the kind of struc- 

 ture to which they contribute. 



Two Contrasted Theories of Multicellular Origin. — In consequence of 

 these two views of the relation of parts to wholes, two general theories 

 of the origin of multicellular organisms have been entertained. Accord- 

 ing to one theory, parts have joined to make wholes; cells have joined 

 to make many-celled bodies. According to the other theory, wholes have 

 been divided into parts. Organisms became complex, then divided into 

 cells whose qualities were dictated by the nature of the whole from which 

 they were produced. 



Which of these theories contains the greater element of truth it is 

 impossible to say. As applied to the origin of metazoa, both have 

 received ardent support from biologists. Both have certain physiological 

 facts in their favor. On the one hand, as a purely logical deduction, it is 

 obvious that the function of an organ is the sum of the things which its 

 component cells do. But that deduction means nothing if the single 

 cells are doing things which are dictated by the whole. On the other 

 hand, it is known from the development of embryos that cells become 

 certain structures because they occupy a certain place among their 

 fellows. But there is no certainty that this is in any sense a consequence 

 of a property of w^holeness in the embryo. The two theories must be 

 left, therefore, with the mere statement of their import, without any 

 attempt to judge between them. 



When, however, one considers the step-by-step consequences of the 

 possible evolution of higher organisms by the one or the other of these 

 general methods, the two concepts rest on different planes. Biologists 

 have usually held that, in the evolution of any line of descent, many 

 branches of the group have arisen, some of which have advanced farther 

 than others. If all of these branches could be collected, they could be 

 arranged in such an order as to give at least a hint of the steps by which 

 the evolution of the most advanced branches had reached their ultimate 

 condition. The less advanced types might, of course, become extinct 

 and so destroy the evidence of the successive stages, and in actual 

 evolution it is certain that such extinction has often occurred. On the 

 chance, however, that some of them have survived, biologists have fre- 

 quently sought among existing relatively simple organisms approximate 

 representatives of the conditions through which the more complex ones 

 have gone in their evolution. The attempt to reconstruct lines of descent 

 by means of series of modern organisms must be done with caution, and 

 no very close correspondence between modern forms and ancestral types 

 can be expected. 



In a reconstruction of the origin of the metazoa by means of a series 

 of modern organisms supposed to represent the evolutionary steps, the 



