TWINNING AS A MODE OF REPRODUCTION 217 



a free-living group with bilateral S3anmetry. Such types 

 as the planarians and Microstomum illustrate alternation 

 of generations very beautifully. For a long time, 

 while relatively young, they reproduce by transverse 

 fission and after a series of fissions reproduce sexually. 

 In the annelids first, and then in the vertebrates, we 

 have the equivalent of the asexual period (characterized 

 by transverse fission) in the process of metameric 

 segmentation. The agamic period is pushed farther 

 and farther back in the life-cycle until in the highest 

 vertebrates it is completed during the first few days of 

 development. The introduction of cases of embryonic 

 twinning in order to supply the missing link in an 

 ideally universal alternation of generations is, therefore, 

 to say the least, gratuitous. Moreover, if this view 

 were carried out to its logical conclusion, we would be 

 led to admit that wherever we find cases of doubling of 

 normally single individuals or parts of individuals we 

 have reminiscences of a lost asexual generation. Such 

 a theory would undoubtedly seem absurd if applied to 

 cases of doubling of tails or doubling of limbs, which 

 begin at relatively late stages of ontogeny. It seems 

 just as absurd to designate the partial fission of the 

 bilateral primordia of a single embryonic axis as a 

 reminiscence of a lost asexual generation. Yet such 

 processes are due essentially to the same factors as is 

 twinning in the armadillo. At the risk of seeming 

 pedantic, therefore, I would once more suggest to 

 biologists the need of caution in assigning phylogenetic 

 values to cenogenetic processes, such as one-egg twin- 

 ning in armadillos and in man, or to experimentally 

 induced twinning in fishes. 



