124 Annals of the South African Museum. 



organs also became simplified by the disappearance of any appen- 

 diculum or swelling at the junction of the free oviduct and the 

 duct of the spermatheca ; and, at the same time, the vas deferens 

 gradually emerged from the wall of the penis and came to lie freely 

 in the body-cavity. 



Now evolution of this kind would proceed most rapidly in the 

 central area where the family first arose, which was probably nearer 

 Ceylon than South Africa ; for, unless the local conditions were 

 unfavourable, the individuals would be most densely crowded 

 towards the centre, and there the competition would consequently 

 be most severe. Towards the periphery of distribution, on the 

 other hand, the individuals would be more sparsely scattered, and 

 the primitive forms would therefore survive, as Taylor has main- 

 tained in dealing with other groups. 



There is, however, another kind of evolution, which proceeds more 

 rapidly towards the limits of the distribution of a group than in the 

 centre ; for as the animals spread in different directions they 

 encounter new conditions to which they have to adapt themselves. 

 This may lead to a profound modification of some of the organs; and 

 if there are great differences in the environment, the various peri- 

 pheral genera may differ more from the ancestral form than do 

 those which still inhabit the central area. But the conditions seem 

 to have been fairly uniform throughout that ancient southern 

 continent; and although the peripheral members of the Acavidae 

 developed differences as they spread in various directions, they 

 remained more primitive than those in the centre of evolution. 



Until about the middle of the Mesozoic era the distribution of the 

 family was probably continuous. And then there came the sea. 

 First in one region and then in another, partly by denudation but 

 chiefly by subsidence, that ancient continent gradually disappeared 

 beneath the encroaching waves. The region extending from the 

 south of India to Madagascar was cut off from Australia on the one 

 hand and from South Africa on the other, and, later, Africa was 

 separated from Brazil. Thus the old home of the Acavidae was 

 divided into four large islands. Henceforth each of the divisions 

 of the family would develop independently, their separation accen- 

 tuating their differences, until the Acavidae were divided into 

 four subfamilies, each inhabiting a different area. 



The subfamily inhabiting the most westerly region, which included 

 Brazil and the adjacent parts of South America, would be on the 

 whole the most primitive, as it was furthest from the centre of 

 evolution. So far as is known, none of the Heliciform members 



