2o6 Report S.A.A. Advancement of Science. 



c. Original Type of the Geometrica-Group. 



It is impossible to say with certainty which of the described 

 forms of the geometrica-gxowp is nearest the original type from which 

 the others may be supposed to have originated. In any ancient group 

 we are not likely to obtain the primary type still living for, on 

 the theory of evolution, it may have been lost in giving rise to new 

 types. In modern groups it is more probable that the primary type is 

 still living on, perhaps unchanged, in the ancestral home, while 

 others, radiating, have been subjected to new environmental forces, 

 and transformed along peculiar directions. 



If we think of any type of the geometrica-gron-^ having the most 

 generalized characters from which the others may have evolved in 

 different directions, tentoria most nearly fulfils these requirements. In 

 this sub-group are gathered together, in more or less incipient stages, 

 all the characteristics which become elaborated to make up the 

 many types of the geometrica-group as we know them. The knobbed 

 form of the dorsal shields is the only character which presents an 

 extreme variation in this sulj-group, but in practically all its other 

 features the sub-group has been taken as the starting point for 

 characters elsewhere more pronounced. 



If we desire an original type, around which all the divergent 

 forms centre, then the verreauxii sub-group most nearly conforms to 

 such. Taking the stages of the various characters, it is easy to see 

 how by variation in one direction we may get such an extreme as the 

 ociilifera type, and, by variation in an opposite direction, we may 

 reach the tentoria type, or the trimeni, and, much more remotely, the 

 geometrica type. 



The most logical course is unquestionably that which regards the 

 tentoria type as the original, for from the variational tendencies repre- 

 sented within the sub-group itself, we can readily conceive how all 

 the different combinations have arisen. 



On the understanding that all these types have originated from 

 some common ancestor, it may reasonably be asked why have not 

 all the forms been transformed ; why have some retained a simple 

 condition in certain of their characters while others have gone on 

 developing? Why have we oculifera, tentoria, and geometrica, repre- 

 senting very different stages of evolution, almost equally abundant 

 at the same time ? If the variations of the group were due to some 

 simple internal force within the organism itself, we should have 

 expected that practically the same level would have been reached 

 by all at the same time, and that the original type would have 

 disappeared in the latter transformations. But this is not the case. 

 Germinal selection as a complex force may possibly have sufificed 

 to produce such varieties, one series of determinants obtaining the 

 upper-hand in one place, and others elsewhere. 



Though the internal processes in the germ plasm might well have 

 run a different course in different, separated colonies, yet it is much 

 more probable that the explanation of the condition of affairs as we 



