130 DR. J. STEPHENSON ON THE MORPHOLOGY, CLASSIFICATION, 



(3) The Objections to the current Explanations. 



I may preface the present section by a brief statement of the 

 reason why I feel a difficulty in accepting the existence of land- 

 bridges as an explanation of the above facts of distribution. 

 It is this. 



Terrestrial Oligochseta are, I believe, a recent group, and some 

 of the genera we have been considering are among the most 

 recent of the earthworms. They have probably arisen in the most 

 recent geological periods. But the general aspect of the fauna of 

 Australia and New Zealand shows that no land connections with 

 Asia have existed during these periods. It is necessary therefore 

 to find other explanations for the existence of so large a common 

 element in the earthworm fauna of these regions. 



(a) Terrestrial Oliogochceta a recent Group. 



The food of earthworms is vegetable mould ; and presumably 

 there were no earthworms in existence until the vegetable mould 

 was present in sufficient quantity to nourish them. We may 

 thus put their rise at some time not earlier than the spread of 

 dicotyledonous plants, which took place during the Cretaceous 

 period. This would limit the evolution of the first earthworms, 

 the differentiation of the several families, and the evolution of the 

 numerous genera of these along lines of descent similar to those 

 we have followed out in the Megascolecinse and other groups, to 

 little more than the Tertiary and Quaternary. 



The recent origin of many of the present-day genera seems also 

 to be indicated by the extraordinary variability of a large 

 number of genera and species. As examples, it may be mentioned 

 that the variability of genital papillae and other markings is a 

 common difficulty of systematists ; that the number of gizzards 

 in the genus Drawida varies fairly widely in many species; that 

 in one and the same genus of Megascolecidaa we may meet with 

 species with testis sacs or with free testes and funnels ; in another 

 with the original (" acanthodriline ") arrangement of the male 

 organs, with the microscolecine reduction, or with the "balantine" 

 reduction (disappearance of anterior prostates, and union of 

 openings of vasa deferentia with the posterior prostatic pores) ; 

 in another, with paired or fused genital orifices ; or with sperma- 

 thecaa varying in number from two to seven pairs or even more; 

 or with seminal vesicles which may vary in number or position 

 or both — indeed these variations of the seminal vesicles are 

 sometimes found within the same species. The consequence 

 is that the generic and specific diagnoses are uncommonly wide 

 as compared with those of other groups. 



Even so, the systematist often has extraordinary difficulty 

 in referring his specimens correctly. He seems to get so many 

 intermediate forms ; in the case of single specimens it is some- 

 times impossible to say whether more ample material would 

 justify the erection of anew species, or would show a range of 



