248 CHAJILES R. STOCKARD 



From what we know of germinal variations and mutations, they 

 do not necessarily give rise to individuals that gradually grade 

 away from the usual type. There may be wide structural breaks 

 between the parent stock and the mutant. On the other hand, 

 we now know that unusual environmental conditions tend to 

 modify the normal course of structural development to varying 

 degrees and give rise to the exact series of defects on which Wild- 

 er's conceptions were based. The present contribution clearly 

 demonstrates the underlying factors and the very probable cause 

 of this orderly series of beings deviating from the normal indi- 

 vidual as monstra in defectu and monstra in excessu. 



The idea is entirely correct that double monsters and twins are 

 due to the same cause as cyclopia. And both may be experi- 

 mentally produced by an identical physical change in the envi- 

 ronment, lowering the temperature. Both conditions also result 

 from a slowing of developmental rate, but one differs from the 

 other because of the difference in the developmental periods dur- 

 ing which the slowing in rate was effective. 



10. THE NECESSITY OF A CONTROLLED OR REGULATED ENVIRON- 

 MENT IN WHICH TO DEVELOP HIGHLY COMPLEX INDIVIDUALS 



From the foregoing considerations it has become evident that 

 normal development of the vertebrate embryo depends acutely 

 upon the stability of certain factors in the environment. Changes 

 in the conditions of moisture, temperature, or oxygen supply are 

 the most frequent causes of embryonic death as well as monstrous 

 development. Any degree of actual dryness is fatal to the ver- 

 tebrate embryo, and sudden lowering of the surrounding tempera- 

 ture and reductions of the oxygen supply interrupt development 

 with the significant consequences discussed above. A normal 

 amount of ordinary food materials is not, however, so acutely 

 necessary for perfect structural expression. The rate of develop- 

 ment under malnutrition is slow, but the depression does not 

 come on suddenly nor is it often sufficiently complete to cause 

 serious structural anomalies. 



Vertebrate animals are faced with the problem of the necessity 

 of a regulated environment in which to develop their eggs into 



