SECTIONAL TRANSACTIONS.— D. 437 



dependent upon some internal factor, or at least to be independent of the 

 environment. 



While some biologists have lately put forward evidence of orthogenetic 

 evolution, orthogenesis has received most sympathetic attention from some 

 palaeontologists. It must be stressed, however, that while palaeontologists 

 occasionally secure reasonably good evidence of lines of evolution, their 

 material is usually too incomplete to afford a basis for useful speculation 

 concerning the causes of evolution. The term ' orthogenesis ' is thus more 

 suitably used to indicate the nature of the phenomena observed than as 

 an explanation of the mechanism. 



Owing partly to the ' mystical ' nature of the supposed internal factors 

 involved in some interpretations of orthogenesis, other terms have been 

 suggested by palaeontologists for similar phenomena. Kitchin wrote of 

 ' programme evolution ' while W. D. Lang formulated the ' Doctrine of 

 Definite Trends ' : it is noteworthy that more recently the term ' trend ' 

 has been used with a similar meaning (apparently independently) by M. M. 

 Metcalfe in his work on Infusoria. 



The following conclusions have been reached by palaeontologists dealing 

 with many different groups : 



(a) In many groups there appears to be definite variation confined to a 

 comparatively small number of trends. 



(b) Similar forms (homoeomorphs) are frequently produced in different 

 lines by ' parallel evolution.' These are not always contemporaneous 

 and in many cases the changes have been thought to be unrelated to 

 any environmental conditions (e.g. certain Brachiopods and Am- 

 monites) ; in other cases the changes were probably adaptive (e.g. 

 the frequently developed ' oxycone ' Ammonites). 



(c) Similar stages in ' unit characters ' may be recognised in different 

 stocks when the whole skeleton is not homoeomorphic. 



(d) In some cases, especially in their later stages, the changes are clearly 

 out of harmony with the environment (e.g. Gryphcea, some Cretaceous 

 Bryozoa) and may lead to extinction. 



Mr. J. Z. Young. — The evolution of the relationship of organisation 

 and environment (11.15). 



Capt. C. Diver. — Polymorphism (11.40). 



Polymorphism is a well-marked expression of variability, but the term 

 polymorphic is often restricted to those cases in which several distinct 

 phases occur together within the same population. This restriction lays 

 some stress on discontinuity and suggests a dividing line which is neither 

 sound in theory nor easily drawn in practice. The variability of an organism 

 must be strictly confined within those limits of biochemical and structural 

 possibility which will still allow the organism to function as a co-ordinated 

 whole ; and species appear to vary in the amount of this potential variability 

 they express at any time. It is necessary first to determine the nature of 

 the factors which provoke and control any particular polymorphic display, 

 before it is possible to estimate its evolutionary significance. 



The problem can be illustrated by numerous examples, but attention is 

 particularly directed to three cases : — Limncea peregra, a very plastic species 

 but not markedly polymorphic in the restricted sense ; Helix (Cepcea), 

 a species showing pronounced polymorphism ; and Plantago maritima, a 

 species which displays polymorphism together with considerable ecological 

 plasticity. 



