FOSSIL PEDIGREES 79 



types of evolution by addition can offer no rational explana- 

 tion. It 'could, perhaps, be accounted for upon the Bateso- 

 nian hypothesis of evolution by loss of inhibition, that is to 

 say, the coincident appearance of convergent characters in 

 collateral lines might be interpreted as being due to a parallel 

 loss in both lines of the inhibitive genes, which had sup- 

 pressed the convergent feature in the primitive or common 

 stock. We say that the convergence might be so interpreted, 

 because the interpretation in question would, at best, be merely 

 optional and not at all necessary; for in the third, or adapta- 

 tional, type of convergence, we shall see instances of parallel 

 modifications occurring in completely independent races, whose 

 morphology and history alike exclude all possibility of heredi- 

 tary connection between them. Hence, even in the present 

 case, nothing constrains us to accept the genetic interpretation. 

 (2) Radical convergence, which Woods styles heterogenetic 

 homoeomorphy, is described by him as follows: "Sometimes 

 two groups of individuals resemble each other so closely that 

 they might be regarded as belonging to the same genus or 

 even to the same species (italics mine), but they have de- 

 scended from different ancestors since they are found to differ 

 in development (ontogeny) or in their palaeontological history; 

 this phenomenon, of forms belonging to different stocks ap- 

 proaching one another in character, is known as convergence or 

 heterogenetic homoeomorphy, and may occur at the same geo- 

 logical period or at widely separated intervals. Thus the form 

 of oyster known as Gryphaea has originated independently from 

 oysters of the ordinary type in the Lias, in the Oolites, and 

 again in the Chalk; these forms found at different horizons 

 closely resemble one another and have usually been regarded 

 as belonging to one genus (Gryphaea) , but they have no direct 

 genetic connection with one another." ("Palaeontology," 5th 

 ed., 1919, p. 15.) Comment is almost superfluous. If even 

 specific resemblance is no proof of common origin, then what 

 right have we to interpret any resemblance whatever in this 

 sense? With such an admission, the whole bottom drops out 



