BY HEBER A. LONGMAX. 13 



as in a case noted by the writer in conjunction with C. T. 

 White,* that most mutations arise when organisms are 

 subjected to a novel environment. Unusual nutritive 

 factors, as recognised by De Vries, are obviously of 

 importance. 



Among instances of modifications in a particular 

 direction as the result of a change in environment we 

 may note three examples : several unrelated species 

 of mangroves tend to develop in their usual habitat 

 asparagoid pneumatophores, and if found on reclaimed 

 land they tend to dispense with these, as pointed out 

 by A. A. Hamilton, t If a change of environment cause 

 a loss, surely the original gain was the result of environ- 

 mental stimuli. In a dry habitat the stomata of plants 

 are found, by comparison, to be fewer than those in allied 

 species in more fertile zones. { In a marine environment 

 such diverse mammals as the walrus, seal, dugong and 

 various Cetaceans exhibit a simplification of the stapes, 

 associated Avith disuse of the organ of hearing. It is 

 difficult to conceive that such changes result from 

 fortuitous germinal variations corrected by natural 

 selection, although that great factor necessarily operates 

 and eliminates the unfit. Many minor structures, 

 however, seem to be of^ no special utility, neither are they 

 disadvantageous, as outlined in my previous paper. In 

 a special environment factors ma}^ operate which bring 

 about an orthogenetic development. But if a varietv 

 of environments is studied, developments in alhed forms 

 will be most complex and may be termed radiogenetic. 

 M. OX'onnell has instanced § an interesting case of 

 orthogenesis in tracing the development of the costae of 

 Perisphinctes. If large numbers of different Ammonites 

 were studied, however, the term orthogenesis would be 

 too restricted to apply. Probably a variety in diet was 

 an important factor in bringing about the extraordinary 

 diversity of the Ammonites, as we have evidence for the 

 operation of this cause in the mollusca to-day. 



♦Longman and White, Proc. Roy., Soc, Qld. XXX., p. 162. 

 fA. A. Hamilton, Proc. Linn. Soc, N.S.W., XLIV., 1919, p. 471. 

 JWarming: Oecology of Plants, 1909, p. 105. 

 §M. O'Connell, Amer. Joum. Science, Dec, 1919. 



