124 DEVELOPMENT AND HEREDITY. _ 



with their differentiated ciUa, oesophagus, pulsating 

 vacuoles, etc. We may therefore conclude that 

 there is no justifiable ground for the assumption of 

 any fundamental physiological difference between 

 the protozoa and the metazoa, other than that differ- 

 ence caused by the greater size of the latter, and its 

 consequent necessities of mechanical support. We 

 are accustomed to consider the universal phenom- 

 enon of an embryonic organism passing from a 

 unicellular to a multicellular stage without thereby 

 inferring a radical physiological change. In the 

 words of Professor Whitman, "the fact that phys- 

 iological unity is not broken by cell boundaries 

 is confirmed in so many ways that it must be 

 accepted as one of the fundamental truths of 

 biology." 



It may be conceded, therefore, that such an ani- 

 mal as Hydra possesses its cell walls chiefly for 

 support, that its protoplasmic continuity is unbroken, 

 and the effects of irritability may be transmitted 

 from any point to all parts of the animal ; in short, 

 that its physiological unity is complete. If we turn 

 from the simpler forms of the metazoa to the more 

 complex forms, we find a different condition of 

 things. A mammal, for example, is composed of a 

 variety of organs which make up an individual. 

 These organs are separate from each other to a 



