330 THE VARIATION OF ANIMALS IN NATURE 



as a process of the same nature as individual senescence. 

 This theory, which was tentatively suggested by Child, was 

 formulated by Beecher (1901) for phyletic changes in orna- 

 mentation in a great number of groups of animals. It 

 has recently been developed with supporting evidence by 

 Fenton (I.e.) in order to explain the modification of the Devonian 

 Brachiopod Spirifer. Fenton (I.e. p. 106 and foil.) adduces 

 as evidence in support of racial senescence in this form the 

 fact that, in ' advanced ' members of the S. orestes ' phratry,' 

 the capacity for repairing the damaged shell, which is well 

 marked in the primitive form, is reduced. He also claims 

 that in Mollusca and Brachiopoda individual susceptibility 

 to environmental effects is increased with age, and that in his 

 Spirifer trends the more advanced members bear the marks 

 of such effects. These physiological trends, he claims, are an 

 index of racial senescence. It must be admitted that some 

 of the evidence brought forward (e.g. by Beecher) is suggestive 

 of a progressive change with age characterised in many groups 

 by similar ' degenerative ' modifications. 



The difficulty we experience in accepting this hypothesis 

 is twofold. (i) As Fenton himself admits, the argument 

 from individual to racial senescence is analogical. We 

 have no proof that racial changes are due to senescence. 

 (ii) There seems to be no correlation between the age of a 

 group and the amount of racial ' senescence.' Historically 

 later stages in a given racial trend are undoubtedly older 

 than earlier ones ; but many forms which are known to be 

 very old historically do not exhibit the degenerative changes 

 that are manifested in a relatively short time in other groups. 

 For example, certain Aspidobranchiate Gastropods are of 

 great antiquity, but forms like Fissurella, Haliotis and Trochus 

 do not exhibit the senescent characters attained in a relatively 

 short time by some Ammonite lineages. 



(3) We have now to consider some special phenomena 

 of excessive or otherwise abnormal growth and some of the 

 attempts to explain them. There is at the present time a 

 large volume of evidence that certain organisms in the course 

 of their evolution have displayed phases of extravagant growth 

 leading to large or over-elaborated structures (' Momentum,' 

 ' Hypertely '). Such phenomena at their most acute or 

 exaggerated expression have been attributed (Lang, I.e. ; 



