408 JOHN BEARD, 



They appear to be adequate to account for the degeneration of 

 the transient apparatus, and they tally exactly with the possible 

 course of events foreshadowed under the hypothesis sketched above. 



The theoretical bearings of the facts of this memoir would only 

 have justice done to them by an extended discussion of the nature 

 of both Metazoan and Metaphytic modes of reproduction in general ^). 

 At the risk of being misunderstood the writer has deemed it fitting 

 to put on record this brief epitome, in order that one day it may not 

 be overlooked that, in describing facts concerning this transient ap- 

 paratus, the factors in the development of Raja pointing to an 

 obvious antithetic alternation of generations were not entirely ignored ; 

 and that it was recognised that the development of the skate fur- 

 nished one instance of such alternation universal for the Metazoa. 



The alternation in different Metazoa would appear to vary widely 

 in its details, and these differences require to be worked out for a 

 great many forms. This statement is intended to indicate that the 

 writer does not assume one universal mode in which the sexual gene- 

 ration replaces the asexual one. The only general thesis to be enun- 

 ciated is that all Metazoan development from the egg is fundamentally 

 an antithetic alternation of generations, and that, directly or indirectly, 

 there is invariably a substitution of organisms. 



Amphioxus is unquestionably an interesting animal, but any sup- 

 position that in its organisation and life-history the key to every 

 puzzling problem in Vertebrate morphology is hidden must be care- 

 fully guarded against. 



Our task would not by any means be finished, even though we 

 were in a position to refer every organ in the Vertebrate back to 

 Amphioxus. There exists a tendency to be satisfied with half-expla- 



account for it are growth in the Ichthyopsida and atrophy of the 

 posterior part of the branchial region in the Amniota. (Comp, de 

 Meuron, in: C. R. Acad. Paris, V. 102, 1886, p. 1403). 



1) If it be admitted that in both kingdoms the reproduction of 

 the higher forms (Metazoa and Metaphyta) is bound up with an obvious, 

 or disguised and reduced, antithetic alternation of generations, we are 

 led to the recognition of a new and fundamental distinction between 

 the higher animals and higher plants. In the former it is the sexual 

 generation w^hich attains higher and higher morphological differentiation, 

 while the asexual generation sutfers a progressive reduction and de- 

 generation. As we ascend the plant-series the conditions are reversed, 

 the sexual generation becoming more and more reduced with the higher 

 evolution of the asexual generation. 



