430 NATURE AND MAN. 



the same effect is produced by multiplication of parts, as is pro 

 duced in ourselves by their concentration in a single apparatus, is 

 altogether conformable to their general type of organization. 

 And it seems to me greatly to strengthen the argument of &quot; inten 

 tion,&quot; that a similar perfection of adaptiveness should be attained 

 by the working-up of the same elementary materials on t\vo 

 different methods of construction, in accordance with the general 

 plan of Articulates and Vertebrates respectively. With regard to 

 those more simple forms of visual apparatus which we regard as 

 inferior or rudimentary, it is to be borne in mind that they prove 

 no less suitable than our own to the requirements of the animals 

 which possess them, and are therefore equally perfect in their kind. 

 All the wants of the Leech, for example, are provided for by its 

 very simply-constructed eyes ; and it would have no use whatever 

 for the elaborately-constructed eyes of the actively-flying Insect, 

 the evolution of the visual organs in the animal series showing a 

 close relation to that of the locomotive apparatus. 



Further evidence of &quot;intelligent design&quot; is supplied by the 

 history of the development of any one of the highest forms of the 

 eye, such as that of the Chick /;/ ovo. For it has been ascertained 

 by the careful study of this process, that the complete organ is 

 the joint product of two distinct developmental actions, taking 

 place in opposite directions, a growing-inwards from the skin 

 and a growing-outwards from the brain : the former supplying the 

 optical instrument for the formation of the visual picture, and 

 the latter furnishing the nervous apparatus on which this is 

 received, and by which its impression is conveyed to the sen- 

 sorium. A hollow, pear-shaped projection is sent out from the 

 division of the brain called the mesencephalon ; the narrowed 

 neck or stalk of which afterwards becomes the optic nerve, whilst 

 its expanded portion, pressed back into a concavity, becomes the 

 retina. At the same time, an inward growth takes place from the 

 skin, at first strongly resembling that which gives origin to a hair- 

 follicle ; a sinking-in of the surface of the dermis or true skin, 

 being accompanied by an increased development of its epidermic 

 cells. This depression deepens into a round pit, the lo.wer part 

 of which expands whilst its orifice contracts, so as to form a 

 closed globular cavity, which is at last completely shut off from 



