GENERATION. 121 



occurs through ova, the animal is said to produce by alter- 

 nate generation.* 



The changes which take place in the impregnated egg in 

 the evolution of the embryo are said to be due to the pres- 

 ence of a peculiar force, recognized as the germ force. f 



Since this is equally pronounced in all ova, and the tissues 

 of the embryo in each case are obtained at its expense, it 

 follows that in comparatively simple animals a large surplus 

 of power is held over and above that actually employed in the 

 maintenance of the adult form, and which is capable of being 

 expressed byjissuration or gemmation in new organisms resem- 

 bling the parent. But when the force is in a measure ex- 

 hausted by the evolution of a complex organization, repetition 

 of the form of the animal can be effected only by the develop- 

 ment consequent upon the union of new sexual elements. It 

 may, therefore, be stated in general terms, that the amount of 

 germ force present in a mature organism is in inverse ratio 

 to the extent to which tissues have been developed, or, in 

 other words, to the degree to which functional labor has 

 been divided. In Infusoria, where this specialization is 



* This terra was introduced by the discoverer of the methoc^ Steenstrup, 

 who entertained the opinion that the first brood of larvae retained the power 

 of producing young, without themselves possessing sexual organs. The 

 following is his original definition: "Alternate generation is the phenomenon, 

 of an animal producing an offspring which at no time resembles the parent, 

 but which on the other hand, itself brings forth a progeny which returns in 

 its form and nature to the parent animal, so that the maternal animal does 

 not meet with its resemblance in its own brood, but in its descendants of the 

 second, third, and fourth degrees of generation." 



The author would consider the phenomena of alternate generation as true 

 larval changes, differing in degree only from those of more direct develop- 

 ment, and producing, as the result of a single impregnation, not one, but it 

 may be, many sexual forms. 



f "Of this force, by whatever name we designate it, whether as the forma- 

 tive or the plastic, or, more explicitly, as the force by which organic matter, 

 in appropriate conditions, is shaped and arranged into organic structures; 

 of this force^, and of those that co-operate with it, we can, I think, only ap- 

 prehend that they are in the completed organism, the same with those which 

 actuated the formation of the original tissues in the development of the 

 germ and of the embryo." (Paget.) 



