WHAT IS EVOLUTION? 



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the flowering and fruiting of sucessively higher and higher branches. 

 Each uppermost branch, under the genial heat and light of direct sun- 

 shine, received in abundance by reason of position, grew rapidly, flow- 

 ered and fruited ; but quickly dwindled when overshadowed by still 

 higher branches, which in their turn, monopolized for a time the pre- 

 cious sunshine. 



But observe, furthermore : when each ruling class declined in im- 

 portance, it did not perish, but continued in a subordinate position. 

 Thus, the whole organic kingdom became not only higher and higher 

 in its highest forms, but also more and more complex in its structure 

 and in the interaction of its correlated parts. The whole process and its 

 result is roughly represented in the accompanying diagram (Fig. 1), in 



Silurian. Devon, and Garb. Mesozoic. Tert'y and Quat. Present. 



Fia. 1. 



which A B represents the course of geological time, and the curve, the 

 rise, culmination, and decline of successive dominant classes. 



The above Three Laws are Laws of Evolution. These three 

 laws we have shown are distinctly recognizable in the succession of or- 

 ganic forms in the geological history of the earth. They are, therefore, 

 undoubtedly the general laws of succession. Are they also laws of evo- 

 lution ? Are they also discoverable in embryonic development, the 

 type of evolution ? They are, as we now proceed to show : 



In reproduction the new individual appears : 1. As a germ-cell a 

 single microscopic living cell.' 2. Then, by growth and multiplication 

 of cells, it becomes an egg. This may be characterized as an aggregate 

 of similar cells, and therefore is not yet differentiated into tissues and 

 organs. In other words, it is not yet visibly organized ; for organi- 

 zation may be defined as the possession of different parts, performing 

 different functions, and all co-operating for one given end, viz., the 

 life and well-being of the organism. 3. Then commences the really 

 characteristic process of development, viz., differentiation or diversifi- 

 cation. The cells are at first all alike in form and function, for all 

 are globular in form, and each performs all the functions necessary 

 for life. From this common point now commences development in 

 different directions, which may be compared to a branching and 

 rebranching, with more and more complex results, according as the 

 animal is higher in the scale of organization and advances toward a 

 state of maturity. First, the cell-aggregate (egg) separates into three 

 distinct layers of cells, called ecto-blast, endo-blast, and meso-blast. 

 These by further differentiation form the three fundamental groups 

 of organs and functions, viz., the nervous system, the nutritive system, 

 and the blood system : the first presiding over the exchange of force 



