Earthworms and Their Allies

The last region into which the world can be divided according to its fauna of earthworms is an Antarctic. I am of distinct opinion that this region is quite necessary in spite of the views of some others. Although the genus Notiodrilus certainly, and Microscolex possibly, extend into the tropical regions of America, Africa, and Australia, these species are but few, and the bulk of the species and of the allied genus Chilota are restricted to the antarctic quarter of the globe; they also extend all over it, that is to say in the southernmost parts of South America, in the Cape region of Africa, in Kerguelen and the Crozet Islands, and in New Zealand, as well as in the Auckland Islands and other neighbouring islands. It is true that I have excluded New Zealand from this region on the grounds that it forms a debateable ground between it and the Indo-Australian. But apart from this part of the world the rest of the territories mentioned should be combined to form the antarctic region.

Having therefore arrived at a mapping out of the world into regions in accord with its earthworm fauna, it is desirable to ascertain what light the facts throw upon the geological and evolutionary questions with which the study of zoogeography deals. The existence of an antarctic region binding together such distant points as South Georgia, the Cape of Good Hope and Kerguelen Island, seems to argue strongly for the former extensions northwards of the antarctic continent so far north as to embrace these several regions of that hemisphere. In view of the facts relating to the danger of sea water to earthworms, to their lack of facilities for migration, other than unassisted locomotion, points which have been dealt with earlier, it is difficult to explain their range in the antarctic hemisphere on other grounds. The very fact that the actual earthworm fauna of New Zealand has led us on the whole to assign it to the Indo-Australian regions shows the inherent uselessness of the current view of zoogeography. For were we to leave the matter here the relationship of New Zealand to the regions of the world which lie to the south of it would not be apparent. However, here as in so many cases there is an antagonism between cut and dried systems and the indications of evolution.

This assumed existence of a former antarctic continent which connected Southern Africa and Southern America as well as various islands has perhaps a further justification in the distribution of the Geoscolecidae. This family is divisible into two well-marked sub-families of which one as has already been mentioned is limited to South America and another practically to Africa (the exceptions being species of the largely aquatic Glyphidrilus), while a third sub-family the Criodrilinae is more widely distributed—again in accordance, one may perhaps assume, with its largely aquatic mode of life. It is also conceivable that the genus Dichogaster is another example pointing the same way. The arguments for regarding this genus as an indigene of the East are not strong. But there is on the other hand no doubt that the Indian Eudichogaster is very closely allied to it. But it is by no means excluded from this argument to suppose that these Trigastrinae owe their likeness to convergence. At any rate there are examples of equally marked convergence which seem to be as nearly proved as can be in another though allied group. The New Zealand Neodrilus is to all intents and purposes a Maoridrilus in which one of the two pairs of spermiducal glands and spermathecae has disappeared. It retains the characteristic alternation in the position of the nephridia of Maoridrilus, and other structural similarities unite the two genera. In the same way species of Microscolex seem as easily derivable from Notiodrilus. Microscolex and Neodrilus are so near that had we no such hint of their origin it would be reasonable to place them in the same genus. They at least show a marked convergence.

It will be noticed therefore that the facts of their distribution agree, as it would appear, with the structure of the terrestrial Oligochaeta. The primitive characters of the genus Notiodrilus are to be seen in the double spermaries and glands appended to the duct, and the corresponding spermatheca, in the absence, or very slight development, of the papillae, so frequent in more specialised genera such as Pheretima, and in the general simplicity of many organs of the body which are more complicated elsewhere. As one would expect with an archaic form this genus is widely ranging, being found in all the principal land masses of the globe except in the Euro-Asiatic continent.

Furthermore geographical facts would at least be not contradictory to the view that this genus, and therefore the terrestrial Oligochaeta generally, originated in the Antarctic hemisphere and that in pushing northwards it has given off various descendants which survive in the various regions of the world. Basing our views of the possibilities of range among earthworms on the actual facts already dealt with, it would seem that the peopling of America from Africa or of Africa from America, if it has occurred, has not taken place through Europe and the north generally. For otherwise we should expect traces of the passage. It is true that we actually have Hormogaster as a possible sign that the Geoscolecidae have passed this way. But that is an isolated case and may be referred to the extension northwards of this particular genus rather than as an indication of a whole migration through those territories. Another conclusion which a collocation of the various facts brought together in this book appears to lead to is that the group of the terrestrial Oligochaeta is relatively speaking a modern one.

Convinced as we must be of the fact that range is only possible by unaided locomotion through continuous land areas, the fact that but few gaps occur in the range of a particular sub-family or lesser group seems to indicate that no great time has elapsed since the specialisation of these different forms. The dependence of earthworms upon vegetable mould also points in the same direction and furnishes an argument for the belief that these animals only greatly increased on the advent of abundant dicotyledonous plants, and perhaps indeed were actually contemporaneous with them.

LIST OF LITERATURE REFERRING
TO EARTHWORMS

In the list given below I am only able to mention a few of the larger works relating to this group. To give anything like a complete list would demand many pages of titles. From the works selected the reader can, if it be desired, find his way to the remaining literature of the group.

Also numerous other works by the above-named authors and by Perrier, Horst, Ude, Lankester, Stole, Pierantoni, Friend, Stephenson, Southern, Goodrich, etc., etc.

INDEX