PROTOZOA TO WORMS 51 



cated and highly developed reproductive organs. By 

 markedly higher forms they certainly grow simpler. 



And here we must notice certain general considera- 

 tions. We found that reproduction in the amoeba 

 could be defined as growth beyond the limit normal to 

 the individual. This form of growth benefits espe- 

 cially the species. The needs and expenses of the in- 

 dividual will therefore first be met and then the bal- 

 ance be devoted to reproduction. Now the income of 

 the animal is proportional to its surface, its expense 

 to its mass and activity. And the ratio of surface to 

 mass is most favorable in the smallest animals.* 

 Hence, smaller animals, as a rule, increase faster than 

 larger ones ; and this is only one illustration of the 

 fact that great size in an animal is anything but an 

 unmixed advantage to its possessor. But muscles and 

 nerves are the most expensive systems ; here most of 

 the food is burned up. Hence energetic animals have 

 a small balance remaining. Now the turbellarian is 

 small and sluggish, with a fair digestive system. With 

 a great amount of nutriment at its disposal the repro- 

 ductive system came rapidly to a high development, 

 and relatively to other organs stands higher than it 

 almost ever will again. 



It is only fair to state that good authorities hold 

 that so primitive an animal could not originally have 

 had so highly developed a system, and that this char- 

 acteristic must be acquired, not ancestral. 



That certain portions of it may be later develop- 

 ments may be not only possible but probable. But 

 anyone who has carefully studied the different groups 

 of worms, will, I think, readily grant that in the stage 



* Cf. p. 35. 



