Structure of Rhizopod Shells. 319 
follow the same laws of circumvolution. From this it follows 
quite definitely that the spiral convolution which occurs in the 
same specific manner 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 tar, 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 nature, 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. 
