162 WORLD-POWER AND EVOLUTION 



This conclusion is of the utmost significance. It agrees with 

 all that we have seen as to variations in butterflies, beetles, fruit 

 flies, aphids, and toads. It shows especial agreement with the 

 highly exact work done by Morgan's pupils on the drosophila fly. 

 Of course the body of evidence thus far available is slight, but it 

 all points in one direction. Apparently the tissues of animals 

 are especially sensitive to extremes of heat or cold during a limited 

 stage in their development. Among insects the most sensitive 

 period seems to be as soon as the eggs begin the process of growth 

 within the body of the mother. Since insects are cold-blooded, 

 the mother's body necessarily shares the changes of temperature 

 occurring in the outside air. Among warm-blooded animals, if 

 the white mouse may serve as a type, the most sensitive period 

 appears to be immediately after birth. While the young are 

 within the mother's body they probably are not influenced by 

 changes in the outside temperature. At birth, however, they are 

 still extremely immature, and their germ plasm is apparently still 

 susceptible of changes. In other words, while inheritance from its 

 parents gives to the young animal most of its characteristics, the 

 conditions of the air immediately after its birth apparently have 

 a certain modifying effect. The new characteristics thus acquired 

 appear to be measurable and to be capable of transmission to 

 offspring. This fact perhaps explains why glacial periods have 

 been times of such rapid evolution. Not only have the sudden 

 extremes of heat and cold then caused the rapid extinction of 

 old species, but the same climatic extremes may have caused rapid 

 and pronounced mutations so that new species and genera were 

 produced in great numbers. 



If these conclusions are true, the animals of extremely hot or 

 cold regions ought to show large numbers of new or unusual 

 characteristics. A. H. Clark has investigated this question in 

 respect to the crinoids, near relatives of the starfishes. He finds 

 that the optimum temperature of the crinoids is not far from 60°. 

 Those living in water near this temperature conform quite closely 



