333 COSMIO l'lIILOSOPUY. [pt.ii. 



Two centuries ago the researches of Ilarvey on generation 

 established the truth that every animal at the ontS6t consists 



simply of a structureless and homogeneous germ. Whether 

 this germ is detached from the parent organism at each 

 generation, as in all the higher animals, or only at intervals 

 of several generations, as for example, in the Ajyhidcs or 

 plant-lice, matters not to the general argument. In every 

 case the primitive state of an animal is a state of relative 

 homogeneity. The fertilized ovum of a lion, for instance, 

 possesses at first no obvious characteristic whereby it can be 

 distinguished from the fertilized ovum of a man, a dog, a parrot, 

 or a tortoise. Each part of the germ-cell is, moreover, as 

 nearly as possible like every other part, in molecular texture, 

 in atomic composition, in temperature, and in specific gravity. 

 Here in two ways we may notice how homogeneity is 

 eventually succeeded by heterogeneity. In the first place, 

 all animal germs are homogeneous with respect to each 

 other, while the animals developed from them present all 

 kinds and degrees of diversity ; and, in the second place, 

 each germ is homogeneous with regard to itself, while the 

 creature developed from it is extremely heterogeneous, The 

 vegetable world exhibits a state of things essentially the 

 same, though less conspicuous in its contrasts. 



Starting from the homogeneous germ, we may follow out 

 a parallel series of differentiations, resulting respectively in 

 molecular rearrangements of chemical elements and in 

 molecular and molar modifications of tissues and organs. 

 The chemical differentiations have been so well and so con- 

 cisely described by Mr. Spencer that I cannot do better than 

 cite the passage entire : — " In plants the albuminous and 

 amylaceous matters which form the substance of the embryo, 

 give origin here to a preponderance of chlorophyll and there 

 to a preponderance of cellulose. Over the parts that are 

 becoming leaf-surfaces, certain of the materials are meta- 

 morphosed into wax. In this place starch passes into one of 



