396 Studies in the Theory of Descent. 



Hydrozoa springs from polypites belonging to 

 quite different families, and in each of these poly- 

 poid families there are species which produce 

 Medusa of another family. 



Similarly, the larvae of the Ophiuroidea (Pluteus- 

 form) among the Echinodermata are not the most 

 closely related in form to those of the ordinary 

 star-fishes, but rather to the larvae of quite a 

 distinct order, the sea-urchins. 



I will not assert that in these two cases the 

 dissimilarity in the form-relationship, or, as I may 

 designate it, the incongruence of the morphological 

 systems, must depend on an unequal rate of phyletic 

 development in the two stages or generations, or 

 that this incongruence can be completely explained 

 by the admission of such an unequal rate of de- 

 velopment : indeed it appears to me probable that, 

 at least in the Ophiurece, quite another factor is 

 concerned that the form-relationship to the larvae 

 of the sea-urchins does not depend upon blood- 

 relationship, but on convergence (Oscar Schmidt), 

 i. e. on adaptation to similar conditions of life. 

 These two cases, however, show that unequal 

 form-relationship of two stages may occur. 



From such instances we certainly cannot infer 

 off-hand that a phyletic force does not exist ; it 

 must first be investigated whether and to what 

 extent such dissimilarities can be referred to unequal 

 phyletic development and, should this be the case, 

 whether deviations from a strict congruence of the 



