74 EFFECTS OF ADAPTATION SEC. 



repeated in succession on all segments; and further, that 

 .{characters which have newly arisen appear afterwards at 

 ian earlier and earlier age, although no advantage is to be 

 discovered in this. 



Although Weismann, even at that period, showed the 

 greatest repugnance to recognising a special force of evolution, 

 and, in my opinion, was quite right in his opposition, yet how 

 can he explain by his theory of the action of sexual reproduc- 

 tion the facts established by himself concerning caterpillars ? 

 And how is it possible to explain definite directions at all in 

 evolution by sexual reproduction, unless every step in the 

 process of modification is demanded by adaptation ? 



CHARACTERS WHICH ARE INESSENTIAL (INDIFFERENT) TO THE 

 LIFE OF THE ORGANISM 



I I once devoted myself for a long time to the study of the 

 siliceous sponges. As is well known, we find in these a 

 degree of variation which is extremely troublesome to the 

 investigator. One form passes into the other, and almost in- 

 numerable, often in the highest degree insignificant, diverg- 

 ences are met with in the parts on which diagnosis depends 

 the siliceous spicules and particles of the skeleton. It cannot 

 possibly be of importance for the success of a given variety 

 whether the spicules have this or that minute character or 

 not. That this is true is proved by the occurrence of many 

 such varieties side by side. As there can be therefore no 

 question here of useful adaptation, the variations under the 

 circumstances can be only ascribed to an extreme sensi- 

 tiveness of the protoplasm to external influences. The " cry- 

 stallisation " in these sponges of the siliceous parts, often so 

 graceful in shape, from the protoplasm is, to an extent most 

 favourable to my ideas, comparable to processes of form- 

 production in inorganic nature, where there can be no question 



