No. 579] PROGRESSIVE EVOLUTION 



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We may legitimately take the nucleated Protozoon 

 cell as our starting point, for, whatever may have been 

 the course of evolution that led up to the cell, there can 

 be no question that all the higher organisms actually 

 start life in this condition. 



We suppose, then, that our ancestral Protozoon ac- 

 quired the habit of taking in food material in excess of 

 its own requirements, and of dividing into two parts 

 whenever it reached a certain maximum size. Here 

 again we must, for the sake of simplicity, ignore the 

 facts that even a Protozoon is by no means a simple 

 organism, and that its division, usually at any rate, is a 

 very complicated process. Each of the daughter-cells 

 presently separates from its sister-cell and goes its own 

 way as a complete individual, still a Protozoon. It 

 seems not improbable that the separation may be due 

 to the renewed stimulus of hunger, impelling each cell 

 to wander actively in search of food. In some cases, 

 however, the daughter-cells remain together and form a 

 colony, and probably this habit has been rendered pos- 

 sible by a sufficient accumulation of surplus energy in 

 the form of food-yolk on the part of the parent render- 

 ing it unnecessary for the daughter-cells to separate in 

 search of food at such an early date. One of the forms 

 of colony met with amongst existing Protozoa is the 

 hollow sphere, as we see it, for example, in Sphcerozoum 

 and Volvox, and it is highly probable that the assump- 

 tion of this form is due largely, if not entirely, to what 

 are commonly called mathematical causes, though we 

 are not in a position to say exactly what these causes 

 may be. The widespread occurrence of the blastosphere 

 or blast ula stage in ontogeny is a sufficiently clear indi- 

 cation that the hollow, spherical Protozoon colony 

 formed a stage in the evolution of the higher animals. 



By the time our ancestral organism has reached this 

 stage, and possibly even before, a new complication has 

 arisen. The cells of which the colony is composed no 

 longer remain all alike, but become differentiated, pri- 



