ROSS ON EVOLUTION. 423 



and are we to consider this large and very complex community 

 of animals as the primordial type? or should we not look rather 

 for a series of types of increasingly complex, and numerous commu- 

 nities of Protozoans leading up to this? and is it not probably that 

 for no inconsiderable period previous to the existence of JEozoon 

 Canadensis^ Protozooa flourished in great numbers and of great 

 size, the sole living occupants of the Earth ? 



The obscurities of embryology may be enlightened wonderfully 

 (though I do not remember ever to have seen it remarked) by 

 studying carefully the embryology of that Class of each Sub-kingdom 

 in which individuals of certain Orders change or partially change 

 their habitat, during the free life of each, from water to land, since 

 in those Orders in which the young are brought forth on land, 

 they must have reached a very much more advanced stage before 

 leaving the egg,- than in those in which the young become free in 

 the water ; and there is thus afforded admirable opportunity of 

 comparing allied forms in the same stage of development, in the 

 one case within the egg, whether intra- or extra-uterine, and in the 

 other while living an active free life in the water ; these last occupy- 

 ing, from every point of view, an intermediate position between the 

 first and the Species in the past history of the Earth in which the 

 mature individuals, living of course in the water, represented the 

 same stage of development. 



All existing Radiates have these remarkable peculiarities, that 

 they are all sessile at some stage of the life of the individual, and 

 that none of them exhibit any of what we call the five senses, but 

 only simple sensation — the common basis of them all. All Radiates 

 that live in the water, when they first leave the planula are free 

 swimmers, and all the higher Orders of them become free again and 

 continue so during their mature life. Land Plants of the higher 

 Orders, which are radiate in structure, and comjDetely sessile in 

 habit, seem almost destitute of sensation, and of the power of 

 motion in their free extremity, and also of the power of digestion, 

 althoui^h Drosera and some other Genera exhibit all three. The 

 sessile Orders o? Articulata and Molluscoida are remarkable for 

 the fact that as each individual reaches the sessile stage it loses its 

 sight, and the tendency to Cephalization, which is a marked 



