Structure of Rhisopod Shells. 319 



follow the same laws of circumvolution. From this it follows 

 quite definitely that the spiral convolution which occurs in the 

 samespecificmanner independently in two quite different groups 

 of organisms having absolutely nothing to do with each other 

 is not founded in the nature of the organisms in question, but 

 has its cause in the circumstance of the external world, and is 

 dependent on statical and mechanical requirements. We 

 have a perfectly analogous case in the statically and mechani- 

 cally adaptive structure of the " substantia spongiosa" of the 

 bones of Vertebrates ; and a series of my own observations 

 make me regard it as very probable that the siliceous rods 

 of a number of spongy Radiolaria are not arranged irregularly, 

 as would appear to be the case at the first glance, but, in part, 

 in accordance with definite laws. The next question which 

 forces itself upon us in considering these results is whether 

 this adaptive structure of animal skeletons has been produced 

 by functional (Roux) or selective (Darwin, Weismann) 

 adaptation. A discussion of the arguments which may be 

 adduced for and against these two possibilities would, how- 

 ever, lead us too far, and pass beyond the bounds of these 

 observations, especially as, without noticing it, we have got 

 upon the question, at present so much in dispute, of the heri- 

 tability of acquired peculiarities. The primary object of the 

 preceding observations was more particularly to indicate the 

 great fertility of a comparative treatment of the enormous 

 abundance of forms of the Protista. The elegant and manifold 

 hard structures of the Rhizopoda, which here particularly come 

 under consideration, are by no means, as is sometimes sup- 

 posed, mere lusus natwce, but even they follow definite laws 

 of structure. Only when we have advanced further in the 

 recognition of the latter by means of more detailed investiga- 

 tions will the morphology of the Protista no longer be re- 

 garded (as is at present unfortunately often the case) as a mere 

 playground of unscientific species-making, but will take its 

 place as of equal importance by the side of the physiology of 

 the unicellular organisms, which is much more cultivated and 

 developed. 



