126 Dr Barry on the Unity of Structure 



heterogeneous or special structure, is universally the same ; and 

 we have already mentioned researches, which seem to warrant this 

 conclusion. 



The direction taken by development, is, however, not the 

 same precisely, in any two animals ; and in different Classes, the 

 direction (type) differs very widely. But of direction, or type, 

 we shall treat more particularly hereafter. 



It has then been shewn, that germs from Infusoria to Man, 

 axe essentially the same, and we know that there are some 

 structural characters, common to all animals in a perfect state, 

 especially to those of the same Class, as, for example, the 

 Vertebrata : there are besides, resemblances between some of the 

 more elaborate structures, in certain of their embryonal phases, 

 and many less wrought out structures, in their permanent con- 

 ditions ; which resemblances are observable, not only between ani- 

 mals included in the same great Class, but also, though more re- 

 motely, between animals belonging to different Classes. 



To sum up these important facts : If the structure of germs 

 has been found in animals at " both ends 1 ' of the animal king- 

 dom, as well as in the intermediate classes, to be essentially the 

 same ; if between the homogeneous masses, forming germinal 

 membranes, there is found no essential difference ; if the pri- 

 mary separation of this membrane into layers (the vegetative 

 layer being always directed towards the yolk), and the subdivi- 

 sions of these layers incipient in the membranal, and completed 

 in the embryonal states are the same in character ; if the for- 

 mation, not of textures only, but of organs also, proceed in the 

 manner just described ; and, above all, if permanent structures, 

 among many of the less elaborate animals, resemble most ob- 

 viously, different degrees of histological and morphological sepa- 

 ration, as presented in the embryonal phases of an individual 

 destined to be more wrought out ; are we not entitled to con- 

 clude, not only that a heterogeneous or special structure arises 

 only out of one more homogeneous or general ; but also that, es- 

 sentially, the manner of the metamorphosis, or metamorphoses,- 

 the manner of development, from the latter to the former state, 

 is universally the same ? 



And are we not then led fairly to the conclusion, that all the 



