THE EVOLUTIONARY CAREER 311 



the tree, for example, synthesizes from CO 2 and OH 2 far more 

 carbohydrate than is required for its reproductive activity and 

 this surplus builds up the mechanical, supporting tree body. 



Mobility has been the main feature of animal evolution. There 

 is nothing that is physiological in nature that is not as efficiently 

 performed by, say, the Infusorian as by the great marine or 

 terrestrial mammals. In the evolution of the structurally complex 

 bodies of the latter animals we see, first, mere increase in size ; 

 second, the development of bodily parts that enable the animal 

 to move ; third, the development of organs analogous to the 

 circulatory vessels of the tree, whereby nutritive materials 

 become distributed throughout a large body and, finally, the 

 evolution of the means of integration that increased bodily size 

 necessitates. 



In the evolution of size the skeleton comes into existence. 

 But in the large Algae, which are floated in water, skeletal structures 

 are unimportant. They become more significant structural parts 

 in the great tree and still more important in the freely movable 

 animal such as the whale or bird. It is curious that certain 

 materials only have been incorporated in skeletal parts : cellulose 

 in the plants and calcium salts in the animals. Why silicates 

 have not been more important features of the plant and animal 

 skeletons than they actually are, and why iron or aluminium salts 

 have not been utilized in the animal skeletons, are curious 

 problems. 



In the evolutionary career, then, the main features have been 

 (Fig. 41): 



(i) Increase of bodily size ; the skeleton ; 



(2) Development of bodily mobility ; 



(3) Development of organs of circulation ; 



(4) The integration of the bodily activities, and a further 

 feature, which we shall study in the " episodes," is the appearance 

 of mere, unessential^ bodily, structural complexity. This we call 

 " excess-value " of transformism (see Section 56). 



104. ON THE MAIN TYPES OF LIFE 

 Neglecting, in the meantime, what we call unessential structural 

 detail we find that the following main organic types have evolved : 

 i. The Thallophytes, including the Algae, the Fungi, Diatoms, 



