MUTATION 71 



fantastic shapes, an excessive number of spines or 

 elaborate sculpturings on the shells as seen among 

 the ammonites, belemnites, and trilobites, or of 

 gigantic size as in the dinosaurs, plesiosaurs, and 

 theromorphs. All of these facts indicate a species- 

 cycle in which these abnormal features were the un- 

 mistakable signs of old age. 



The reproductive period of a species when mutants 

 are being thrown off, as of an individual, may ex- 

 tend over a considerable period of the whole cycle, 

 or it may be confined to a relatively small segment. 

 It is possible that in the evening primrose de Vries 

 may have caught a plant passing through the crucial 

 period of species-reproduction. 



Another reason why so few mutations have as yet 

 been seen, is because the majority of organisms are 

 not, during the short span of human observation, 

 in the reproductive part of their cycles. When it is 

 remembered that accurate observation with this 

 object in view has extended over only a brief period, 

 insignificant in comparison with the vast geologic 

 stretches of time concerned in species-building, the 

 marvel is that so much, rather than that so little, has 

 been seen. 



A further suggestion in connection with the pos- 

 sible sources of mutation is that mutations may be 

 the results of hybridization, appearing as Mendelian 

 recessives after crossing. As a matter of fact, Spren- 

 gel's Chelidonium ladniatum, already cited, when 

 crossed with Chelidonium majus, behaves according to 

 such an expectation. This phase of the question, 



