DEVELOPMENT. 173 



ing forms of life exhibit this progressing differentiation from 

 the environment. 



In the first place, it is illustrated in structure. Advance 

 from the homogeneous to the heterogeneous, itself involves 

 an increasing distinction from the inorganic world. Passing 

 over the Protozoa, of which the simplest probably dis- 

 appeared during the earliest stages of organic evolution, and 

 limiting our comparison to the Metazoa, we see that low 

 types of these, as the Ccelentcrata, are relatively simple in 

 their organization; and the ascent to organisms of greater 

 and greater complexity of structure, is an ascent to organisms 

 which are in that respect more strongly contrasted with the 

 structureless environment. In form, again, we see 



the same truth. An ordinary characteristic of inorganic 

 matter is its indefiniteness of form; and this is also a cha- 

 racteristic of the lower organisms, as compared with the 

 higher. Speaking generally, plants are less definite than 

 animals, both in shape and size admit of greater modifica- 

 tions from variations of position and nutrition. Among ani- 

 mals, the simplest Ehizopods may almost be called amor- 

 phous : the form is never specific, and is constantly changing. 

 Of the organisms resulting from the aggregation of such 

 creatures, we see that while some, as the Foraminifera, as- 

 sume a certain definiteness of form, in their shells at least, 

 others, as the Sponges, are very irregular. The Zoophytes 

 and the Polyzoa are compound organisms, most of which 

 have a mode of growth not more determinate than that of 

 plants. But among the higher animals, we find not only that 

 the mature shape of each species is very definite, but that the 

 individuals of each species differ little in size. A 



parallel increase of contrast is seen in chemical composition. 

 With but few exceptions, and those only partial ones, the 

 lowest animal and vegetal forms are inhabitants of the 

 water; and water is almost their sole constituent. Desic- 

 cated Protophyta and Protozoa shrink into mere dust; and 

 among the Acalephes we find but a few grains of solid matter 



