DEVELOPMENT. 149 



the point of departure common to all organisms, is the most 

 generalized of figures ; and one that is, under various circum- 

 stances, assumed by inorganic matter. V/hile the incipient 

 organism is spherical, it is not only like many particular in- 

 organic masses ; but it is like the rest, in the sense that it has 

 the shape which would result, were all their irregularities 

 averaged. But as it develops, it loses all likeness to inor- 

 ganic objects in the environment ; and eventually becomes 

 distinct even from all organic objects in its environ- 

 ment. In specific gravity, the alteration, though not 

 very marked, is still in the same direction. Development 

 being habitually accompanied by a relative decrease in the 

 quantity of water, and an increase in the quantity of consti- 

 tuents that are heavier than water, there results a small aug- 

 mentation of relative weight. In power of maintain- 

 ing a temperature above that of surrounding things, the 

 differentiation from the environment that accompanies deve- 

 lopment, is marked. All ova are absolutely dependent for 

 their heat on external sources. Like inorganic bodies, they 

 gain or lose heat according as neighbouring bodies are colder 

 or hotter. The mammalian young is, during its uterine life, 

 dependent on the maternal heat ; and at birth has but a par- 

 tial power of making good the loss by radiation. But as it 

 advances in development, it gains an ability to maintain a 

 constant temperature above that of surrounding things : so 

 becoming markedly unlike all surrounding things, save or- 

 ganisms of allied nature. Lastly, in self -mobility this 

 increasing con crast is not less decided. Save in a few aber- 

 rant tribes, chiefly parasitic, we find the general fact to be, 

 that the locomotive power, totally absent or very small at the 

 outset, increases with the advance towards maturity. The 

 more highly developed the organism becomes, the stronger 

 grows the contrast between its activity and the inertness of 

 the objects amid which it moves. 



Thus we may say that the development of an individual 

 organism, is at the same time a differentiation of its parts 



