MULTIPLICATION IX ANIMALS 489 



energy; yet in any species of organism that produces these expensive 

 structures or renders these expensive services every individual gets the 

 full benefit of the additional expenditure at the very beginning of his 

 career. We might say that an organism, apart from all other considera- 

 tions, comes to be able to do all its life's work well just in proportion as 

 its parents have guarded its youth and have given it a good start. In 

 doing things for posterity an organism is thus merely repaying to the 

 species what was done for it in the past. 



MULTIPLICATION IN ANIMALS 



I. Cell division 



One-celled animals 



Multiplication 

 Many-celled animals 

 Growth 

 Development 

 Healing and repair 

 Grafting 



Possibilities within species 

 Possibilities from one species to another 

 Limitations 

 Of "taking" 

 Of staying 

 * Regeneration 

 Possibilities 

 Limitations 

 Abnormal growths 

 Tumors ; cancer 



2. Spores in animals 



Protozoa 



3. Sexual reproduction 



Conjugation 



Protozoa 

 External fertilization 



Among invertebrates (living in water) 



Fishes 



Some batrachians 

 Internal fertilization 

 Cartilaginous fishes 

 Some batrachians 

 Insects 



