STRUCTURE. 195 



While the first named of these facts are displayed in 

 every Metazoon, and while the last named are visible only 

 in Metazoa of considerably developed structures, a gradual 

 transition is shown in intermediate kinds of Metazoa. Of 

 this transition it remains to say that it is effected by the 

 progressive development of auxiliary appliances. For 

 example, the primitive foot-cavity is a sac with one opening 

 only; then comes a second opening through which the 

 waste-matter of the food is expelled. The alimentary canal 

 between these openings is at first practically uniform; after 

 wards in a certain part of its wall arise numerous bile-cells; 

 these accumulating form a hollow prominence; and this, 

 enlarging, becomes in higher types a liver, while the hollow 

 becomes its duct. In other gradual ways are formed other 

 appended glands. Meanwhile the canal itself has its parts 

 differentiated : one being limited to swallowing, another to 

 triturating, another to adding various solvents, another to 

 absorbing the prepared nutriment, another to ejecting the 

 residue. Take again the visual organ. The earliest form of 

 it is a mere pigment-speck below the surface. From this 

 (saying nothing here of multiple eyes) we rise by successive 

 complications to a retina formed of multitudinous sensory 

 elements, lenses for throwing images upon it, a curtain for 

 shutting out more or less light, muscles for moving the 

 apparatus about, others for adjusting its focus; and, finally, 

 added to these, either a nictitating membrane or eyelids for 

 perpetually wiping its surface, and a set of eyelashes giving 

 notice when a foreign body is dangerously near. This process 

 of elaborating organs so as to meet additional requirements 

 by additional parts, is the process pursued throughout the 

 body at large. 



Of plant-structures, concerning which so little has been 

 said, it may here be remarked that their relative simplicity is 

 due to the simplicity of their relations to food. The food of 

 plants is universally distributed, while that of animals is 

 dispersed. The immediate consequences are that in the one 



