THE FACTORS OF ORGANIC EVOLUTION. 107 
(2) 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. lv. 
