2G OUTLINES OF EVOLUTIONARY BIOLOGY 



however, which are really composed of opal, do not consist either 

 of shapeless masses or of geometrical crystals, but assume beauti- 

 fully symmetrical forms which vary with each particular kind of 

 organism and which are wholly different from any forms occurring 

 in the inorganic world. The same is true of those somewhat 

 more highly organized members of the animal kingdom, the 

 siliceous sponges, whose skeletons consist of spicules of opal 

 (Fig. 88), often of most beautiful and characteristic shape, and each 

 one as a rule formed by the activity of the living protoplasm 

 within the compass of a single cell. 



Whilst many unicellular organisms and many sponges thus 

 manufacture for themselves skeletons of a siliceous character, 

 others, living perhaps in the same water, secrete skeletons which 

 are composed of carbonate of lime. Such, for example, are the 

 Forarninifera, the dead calcareous shells of which (Fig. 4) 

 accumulate to-day at the bottom of the deep sea in many places 

 in the form of ooze, while in the Cretaceous period of the earth's 

 history they played the principal part in building up the chalk 

 cliffs on the south coast of England. Each of these microscopic 

 shells, which are often of extreme beauty and frequently 

 ornamented with elaborate patterns, is formed as a Drotective 

 envelope by a simple protoplasmic organism closely resembling 

 an Amoeba. 



It is evident, then, that we must attribute to living proto- 

 plasm a very remarkable power of selection or choice, for it is 

 able, as it were, to pick out certain materials from its environ- 

 ment for its own purposes and to reject others. We have already 

 seen this in the case of the selection of food materials by the 

 Amoeba ; we see it again in the selection of the materials for 

 skeleton formation in other simple organisms. The fact that 

 one organism will select silica while another selects carbonate of 

 lime from the same sample of sea water and for the same purpose, 

 must correspond to some deep-seated difference in the protoplasm 

 of which they are composed, and illustrates very well the diverse 

 potentialities of this remarkable substance. 



