335 



Indeed, given time enough, the power which an organism 

 possesses of altering its bodily structure in accordance with new 

 demands on the part of the environment seems, as we have 

 already pointed out, to be almost without limits. 



This plasticity is illustrated in the most striking manner in 

 cases where the organism has been removed from what may be 

 regarded as the normal environment of the group" to which it 

 belongs, and to which the great majority of the group are adapted, 

 and come to live under new 

 and very different conditions. 

 Thus it is with the aquatic 

 and aerial mammals, which, 

 in encroaching upon the 

 domains of the fishes and 

 birds, have, by convergent 

 evolution, come to resemble 

 these in bodily form. 



Wherever we turn we find 

 fresh illustrations of the same 

 principle. At great depths 

 of the ocean the conditions of 

 life are very different from 

 those which obtain in shal- 

 low water, and we find the 

 animals which inhabit these 

 abysses modified accordingly. 

 Fig. 168 represents two deep 

 sea sponges obtained by the 

 " Challenger " expedition ; 

 Cladorldza longipinna from a 

 depth of 3000 fatboms in the North Pacific and Axonidenna miralile 

 from a depth of 2250 fathoms in the South Pacific. It will be seen 

 at once that the form assumed by these sponges is very unusual 

 and quite unlike that exhibited by their shallow water relatives. 

 The great majority of the members of the group of sponges (the 

 Tetraxonida) to which they belong are indeed by no means 

 remarkable for symmetry of shape, but these two are beautifully 

 symmetrical, their form at once suggesting that of a parachute, 

 with a small conical body fringed by long radiating processes 

 surrounding a central root-like projection. This " Crinorhiza 

 form," as it is termed, is obviously an adaptation which serves 



FIG. 168. Two Deep Sea Sponges, 

 exhibiting the Crinorhiza Form. 

 A. Cladorhiza longipinna; B, Axoni- 

 derma mirabile ; nat. size. (After 

 Ridley and Dendy in " Challenger " 

 Reports.) 



