THE INSTABILITY OF THE HOMOGENEOUS. 427 



display no differentiations of surface; the several parts of 

 their surfaces being subjected to no definite contrasts of con- 

 ditions. Ciliated spheres such as the Volvox have no parts 

 of their periphery unlike other parts ; and it is not to be ex- 

 pected that they should have; since, as they revolve in all 

 directions, they do not, in traversing the water, permanently 

 expose any part to special conditions. But when we come to 

 organisms that are either fixed, or while moving preserve 

 definite attitudes, we no longer find uniformity of surface. 

 The most general fact which can be asserted with respect to 

 the structures of plants and animals, is, that however much 

 alike in shape and texture the various parts of the exterior 

 may at first be, they acquire un likenesses corresponding to 

 the unlikenesses of their relations to surrounding agencies. 

 The ciliated germ of a Zoophyte, which, during its locomo- 

 tive stage, is distinguishable only into outer and inner tis- 

 sues, no sooner becomes fixed, than its upper end begins to 

 assume a different structure from its lower. The disc-shaped 

 gemmcB of the Marchantia, originally alike on both surfaces, 

 and falling at random with either side uppermost, imme- 

 diately begin to develop rootlets on the under side, and 

 stomata on the upper side: a fact proving beyond ques- 

 tion, that this primary differentiation is determined by this 

 fundamental contrast of conditions. 



Of course in the germs of higher organisms, the meta- 

 morphoses immediately due to the instability of the homo- 

 geneous, are soon masked by those due to the assumption of 

 the hereditary type. Such early changes, however, as are 

 common to all classes of organisms, and so cannot be ascribed 

 to heredity, entirely conform to the hypothesis. A germ 

 which has undergone no developmental modifications, con- 

 sists of a spheroidal group of homogeneous cells. Univer- 

 sally, the first step in its evolution is the establishment of a 

 difference between some of the peripheral cells and the cells 

 which form the interior — some of the peripheral cells, after 

 repeated spontaneous fissions, coalesce into a membrane; 



