292 BIOLOGT. [Book tii. 



oxygen is indispensable to the nutrition, and'consequently to the 

 life of organised beings and their anatomical elements. Truly, 

 , we may say of oxygen, as the ancients did of the air in general, 

 that it is the pahulum vitce. 



Like the vegetal anatomical elements, the animal elements can 

 only develop and multiply within certain limits of temperature. 

 There are, for animal and vegetal development, thermic cardinal 

 points which seem very near each other. The temperature of 

 from 30 to 35 degrees seems to be one of the most favourable, 

 since it is this which animals, with warm blood, maintain, in spite 

 of the thermic variations of the exterior medium. In his experir 

 ments in embryogeny, M. Dareste has observed that, elevated 

 temperatures constantly determine, first an acceleration of the 

 evolutive phenomena, then their premature stoppage ; whence 

 nanism. On the contrary, lower temperatures much retard the 

 progress of development; they even stop the evolution, and, dp 

 not permit the embryon to advance beyond a certain period.^ 



The influence of the seasons is attended by analogous results. 

 In man, and most animals, the maximum of grow this in summer, 

 and the minimum in winter. 



If, by means of an artificial hatching apparatus, we, as M. 

 Dareste has done, limit the influence of the source of heat to a 

 fixed point of the embryon, we can, by varying the position of 

 the ovum, obtain all the types of simple monstrosities described 

 in the treatises on teratology. 



The rapidity of nutritive exchanges, and consequently of 

 growth, is closely connected with the abundance of liquids, within 

 the living tissues. In ligneous vegetals, the fibres have less 

 vitality in proportion as they harden. Plants, with loose tissues, 

 loaded with juices, grow more quickly than vegetals with dense 

 tissues. 



In the same way, amongst animals, the histological elements 

 are less impregnated with humidity in proportion as the animal 



^ ExposS des Titres.et des Travaux Sdentifijues.de-M^ Camille Dareste. 

 Paris, 1868. 



