232 Animal Life and Intelligence. 



Let us notice that, \vide as the variations are, they are 

 to a large extent hedged in by physical, chemical, and 

 organic limitations. We have already seen that the size 

 of cells is to a large extent limited, because during growth 

 mass tends to outrun surface ; and because, while disrup- 

 tive changes occur throughout the mass, nutriment and 

 oxygen must be absorbed by the surface. This is a 

 ^physical limitation. Since the products of cell-life and 

 cell-activity are chemical products, it is clear that they can 

 only be produced under the fixed limitations of chemical 

 combination ; and though in organic products these limi- 

 tations are not so rigid as among inorganic substances, 

 still that there are limitations no chemist is likely to 

 question. The organic limitations are to the varied, but 

 not very numerous, modes of protoplasmic activity. 



Probably, even at the threshold of metazoan life, such 

 variations did not affect only individual cells, but rather 

 groups of cells. In other words, the differentiation was at 

 once and primarily a tissue-differentiation. What do we 

 know, however, about the primitive tissue-differentiation of 

 the earliest metazoa ? Hardly anything. We may fairly 

 suppose that the first marked difference to appear was 

 that between the outside and the inside. In the formation 

 of an embryo this is the first differentiation we notice. 

 From the beginning of segmentation or, in any case, very 

 early, the outer-layer cells become marked off from the 

 inner-layer cells. The next step was, perhaps, the forma- 

 tion of the mid-layer between the outer and inner. But 

 how further differentiations were effected we really do not 

 know, though we may guess a little. This, perhaps, we 

 may fairly surmise that fresh differentiations presupposed 

 previous differentiations, and formed the basis of yet further 

 differentiations. Thus calcified cartilage presupposes car- 

 tilage, and leads up to the formation of true bone. In all 

 this, however, we are very much in the dark. We can 

 watch, always with fresh wonder, the genesis of tissues in 

 the development of the embryo ; but we do not at present 

 know much of the mode of their primitive genesis in the 



