214 THE PHYSIOLOGY OF TWINNING 



reproduce much in the same way for some generations 

 until finally a generation of cercarias is produced. 

 These find their way out of the snail and into the liver of 

 the sheep, where each slowly transforms itself into an 

 adult fluke. 



There are several species of paedogenetic insects in 

 which reproduction takes place in the larval condition. 

 Paedogenesis is recognized in some of the vertebrates 

 also, as in the classic Axolotl and probably in the perenni- 

 branchiate urodeles. In fact paedogenesis grades over 

 almost imperceptibly into full adult reproduction. In 

 very early paedogenesis the reproduction is of necessity 

 parthenogenetic, but in later paedogenesis regular syn- 

 gamous reproduction takes place. Just as there is a 

 gradation between agametic and gametic reproduction, so 

 we have a graded series of stages ranging from extremely 

 early paedogenesis to late adult syngamous reproduction. 

 Parthenogenesis is evidently a phase of gametic reproduc- 

 tion and its incidence is capricious; we find it here and 

 there throughout the animal kingdom from Protozoa 

 to Chordata. It is not to be thought of as a reversion 

 to a primitive mode of reproduction but rather as an 

 evidence of racial senescence. 



THE COMMON FEATURE OF ALL REPRODUCTIVE 

 MODES 



The one connecting link between the various modes 

 of reproduction is the principle of physiological isolation. 

 So long as the individual is completely integrated in all 

 of its parts it will not reproduce, but if for any reason 

 the integrative forces that hold together the various 

 parts into a single organism weaken, either a general or 



