THE FACTORS OF ORGANIC EVOLUTION. 



107 





(<5) Herbst * has shown in a series of interesting ex- 

 periments that by the use of various chemical substances 

 the development of echinoderms may be profoundly 

 modified. For example, in sea water deficient in cal- 

 cium chloride, or in which there is an excess of potas- 

 sium chloride, the Pluteus larva, instead of developing 

 calcareous spicules and the long ciliated arms which 

 give the normal larva an angular, easel-shaped appear- 

 ance, remains rounded in shape much like the larva of 

 Balanoglossus, in which no spicular skeleton is developed. 

 The withdrawal, therefore, of certain normally present 

 substances from the environment may profoundly modi- 

 fy the final result. But in this case, as in the other, it is 

 absolutely certain that the calcareous spicules were pre- 

 determined in the egg cell, although in the absence of 

 calcareous matter from the water those spicules could 

 not be built the plan was there, but the building ma- 

 terial was lacking. 



Such modifications resulting from unusual conditions 

 of pressure, temperature, density, nutrition in fact, any 

 alteration of the chemical or physical environment may 

 appear in any stage of development from the unseg- 

 mented egg to the adult condition, but it must not be 

 supposed that the entire development can be reduced to 

 such factors. Loeb argues that we do not inherit our 

 body heat from our parents because it depends upon 

 certain chemical processes ; but is it not absolutely cer- 

 tain that we inherit a certain protoplasmic structure 

 which determines those chemical processes, and hence 

 the body temperature ? To assume that extrinsic causes 

 determine whether there shall hatch from an egg a 

 chicken or an eagle is the sheerest nonsense. The 

 study of extrinsic factors in relation to inheritance will 



* Zeit. wiss. Zool., Bd. Iv. 



