74 THE EVOLUTION OF MAK. 



bability, that all these Plant-animals descend from a 

 common and very simple parent-form, the structure of 

 which resembled that of the ascula in essential points 

 (Figs. 182, 183, p. 68). The uniaxial outline of the ascula 

 and the gastrula is usually retained by the Sponges, while 

 in most Sea-nettles (Accdephce) transverse axes have been 

 differentiated in the course of further evolution, thus giving 

 rise to a characteristic radiate structure with a pyramidal 

 general outline. 



In distinction from this predominant radiate outline of 

 Plant-animals, a marked bilateral general outline is de- 

 veloped from the first in the second offshoot from the 

 gastrula, in the Worms. As the radiate form is marked by 

 adaptation to an adherent mode of life, so is the bilateral 

 form by adaptation to certain definite acts of free loco- 

 motion. The constant direction and carriage of the body 

 which would be maintained in this mode of free locomotion, 

 conditioned the two-sided, or bilateral outline of the 

 symmetrical Worms. Even the parent-form of the latteu, 

 the Primitive Worm (Prothelmis) must have acquired this 

 character, and thus have become distinguished from the 

 uniaxial parent-form of the Plant-animals. In this simple 

 mechanical impetus, in the defined free locomotion of the 

 Worms, on the one hand, and in the stationary mode of 

 life of the earliest Plant-animals on the other, we must look 

 for the efficient cause which produced in the one the bi- 

 lateral or two-sided, in the other the radiate outUne of the 

 body. The former, the bilateral outline, has been inherited 

 by the human race from the Worms. 



Except through the Gastrsea, the common parent-form 

 of Plant-animals and Worms, the human race is, therefore, 



