228 Variation and Correlation in the Earthivorm 
In position and extent of the clitellum there is essential agreement between 
the Ann Arbor form and the tj^pical L. herculeus. In size and number of somites, 
however, there is a striking discrepancy. It would appear that the earthworms 
typical for this region approach towards a dwarf condition of the type of the 
species, being on the average only a little more than half as long and having about 
40 fewer somites. The typical L. herculeus, according to the above figures, has for 
the single somite an average length of 2 mm., as against 1"34 mm. for our sample. 
Applying equations (i) and (ii) (p. 219) we find the mean length of an array of 
worms each having 180 somites, 216'9 mm. instead of 360 mm., as given for 
L. herculeus, and for the mean number of somites for an array having a length 
type of 360 mm., 159"6 as against 180. 
Is this Ann Arbor form to be regarded as a true dwarf variety, or is the small 
size to be explained in some other way ? Several possible explanations occur to 
one but none of them seems adequate. In the first place it might be maintained 
that the specimens in our sample were young, and had not completed their growth. 
This is, undoubtedly, true for some of the specimens in the sample. The earth- 
worm continues to add somites for a long period, possibly throughout its life. If 
such addition does occur throughout the life of the worm it must, after a time, be 
at a very slow rate. This is evidenced by the fact that in the present sample 
there is no tendency towards extreme extension of the range at the upper end. 
The form of the frequency polygon for these worms, as has been pointed out 
above, indicates that the majority have either stopped growing or are growing at 
a very slow rate. 
Another possibility is that in the collection of these worms an unconscious and 
unavoidable selection was made on the basis of size. This might arise in the 
following way. Suppose it were a habit of very large (old) worms not to leave the 
burrows on rainy nights. Then it would necessarily result that any collection made 
in the way the present one was made, would contain no or few very large worms. 
Now it is possible that this is the case, but it seems to me hardly probable, and 
for the following reason : in the large number of earthworms which have been 
collected for class-work in this laboratory during the last five years, including 
several thousand individuals at least, specimens have not been found of twice the 
size of the average of the present collection. Furthermore worms collected by 
digging are of no larger size than those collected in the way described in this 
paper*. Finally no student of earthworm habits has recorded, so far as I am able 
to find, any tendency towards a habit of the sort mentioned. 
Again it might be thought that the relatively small size of the worms in this 
sample as compared with the type of the species was due to the method of killing 
and fixing. That this cannot be the case is shown, first by comparison with living 
* These last two facts also argue strongly against the first point raised ; viz., that the worms in the 
present sample are all young specimens. Surely in five years collecting a fair number at least of old 
worms would be found. 
