292 Proceedings of the Royal Physical Society. 



It- will now become apparent that it is no easy task to 

 trace the direct line of the ascent of man, from a low uni- 

 cellular organism up to his present position of superiority 

 over his fellow- creatures. On the earlier stages of the 

 general advance of life, Professor Cleland makes the follow- 

 ing pertinent remarks : — 



"We pass from masses indeterminate and spheroidal in 

 shape, minute beings moving in water, barely visible, or 

 seen only with the microscope, mere isolated living cor- 

 puscles, beings scarcely, if at all, exhibiting separate organs, 

 to others more elongated, with the mouth towards one 

 extremity, which, in the event of the animal being fixed like 

 a plant, is the extremity standing out free in the water, and 

 in the event of its moving about, is placed foremost, so as to 

 come in contact with food as it moves along. Then, in such 

 an animal, when not fixed but moving from place to place, 

 there is one surface turned to the ground and another toward 

 the light, and having therefore very different relations. 

 Thus, you observe that in the mere surroundings of animals 

 of the simplest kinds there is a rationale and an excitant to 

 be found for the origin, first of starry or cylindrical sym- 

 metry out of the original Qg^ shape, then for the origin of a 

 head and a tail end, and subsequently of a ventral and a 

 dorsal aspect, with, as a consequence, so long as the growth 

 is equal, bi-lateral symmetry, or a right and a left side. I 

 may add that any advantage of one side over the other 

 in growth would turn the body to one side, and, by being 

 continued, originate a spiral — a form which abounds in the 

 animal kingdom. Another series of complications sets in, 

 connected with the relation of different parts to the surface. 

 The superficial part, coming into relation with the world 

 around, becomes what Bichat called the animal sphere, de- 

 voted to sensation and locomotion ; while the deep part 

 round the digestive cavity becomes separated as the vegetal 

 sphere, and the body-cavity makes its appearance between. 



" As the axis of the animal further elongates, we next find 

 the phenomenon called segmentation developing, that is to 

 say, a repetition of similar parts in linear series, producing 

 the sort of arrangement with which every one is to some 



