312 TIIK EVOLUTION OF MAN. 



Comparative Anatomy, like every other science, is endless; and therefore 

 the endlessness of the accumulation of materials would never allow men, if 

 they complied with this dem.-md, to reap any harvest from this field. But, 

 further than this, history teaches as clearly, that no age in which scientific 

 inquiry has been active, has been able so to deny itself, as, setting the goal 

 of it* researches in -the futnre, to refrain from drawing conclusions for itseK 

 from its larger or smaller treasury of observations, and from trying to fill tin 

 gaps with hypotheses. It would, indeed, be a hopeless proceeding, if, in 

 oi'cier to avoid lo--.in.fe any part of our possessions, we should refuse tc 

 acquire any possessions whatever." KARL ERNST RAKR (1N19). 



AMONG the vegetative organs of the human body, to the 

 development of which we now turn our attention, the intes- 

 tinal canal is the most important. For the intestinal tube 

 is the oldest of all the organs of the animal body, and 

 carries us back to the earliest time of organological differ- 

 entiation, to the first period of the Laurentian Epoch. As 

 we have already seen, the result of the first division of 

 labour in the homogeneous cells of the earliest many-celled 

 animal body must have been the formation of a nutritive 

 intestinal canal. The first duty and the first need of every 

 organism is self-support. This task is accomplished by the 

 two functions of nutrition and of the covering of the body. 

 When, therefore, in the primaeval collection of homogeneous 

 cells (Synamcebium), of the phylogenetic existence of which 

 we yet have evidence in the ontogenetic developmental 

 form of the mulberry-germ (Morula), the several members 

 of the community began to divide the work of life, they 

 were first obliged to engage in two separate tasks. One 

 half modified into nutritive cells, enclosing a digestive 

 cavity, the intestinal canal ; the other half, on the contrary, 

 developed into covering cells, forming the outer cover- 

 ing of this intestinal canal, and, at the same time, of the 

 whole body. Thus arose the first two germ-layers : the 



