xciv Journal of Proceedings.
fallen leaves and small twigs underneath the high hawthorn bushes
which keep the ground below slightly moist. In the variety the animal
itself seems to be of the usual colour, but the shell, instead of being a
horny brown, is transparent, and coloured white with a tinge of green
and a reddish apex. Specimens intermediate in colour occur, but are
not common.
On a subsequent visit, on January the 18th, 1878, but in very mild
weather, I discovered, at the same spot and all near together, three
specimens of Bulimus obscurus, var. alba—another exceedingly rare
variety, although the type-form is pretty common at the spot in ques-
tion, and in other suitable spots throughout the county. I have not,
since this visit, succeeded in finding more than one other specimen, and
that a broken one, so I conclude it is very scarce there.
On April the 12th, 1879, whilst collecting specimens of the former
variety, I obtained a single individual of Cochlicopa tridens, var.
crystallina—another very uncommon variety of a shell which, though
perhaps not rare in Essex, is certainly not common, and is only to be
met with in a limited number of localities. On account of the similarity
of the two varieties I did not discover what I had obtained until I had
reached home, but, as I have never on any other occasion seen this
variety there, I conclude that it is very scarce. The type exists in the
locality, but in very limited numbers indeed.
This, then, makes altogether three white varieties of great rarity—in
fact, so far as I am aware, there are not more than a dozen recorded
localities for any one of them in England, yet here, on an area of ground
covering less than half-an-acre, they are all three to be met with, and
this fact suggests some interesting lines of enquiry as to the general
cause of white varieties in Nature, and as to the special causes of the
phenomenon in the present instance. Is it due to the lack of some
particular constituent of the air or soil, or of the plants native to the
place ? And if some local peculiarity is the determining cause of the
variation, why are not all instead only some of the shells white? The
situation in which these shells live must be extremely dry, inasmuch as it
consists simply of a high mound of earth with a moat round most of it.
I may say that, of the other shells inhabiting this spot, some belong to
species which are decidedly uncommon in the immediately surrounding
neighbourhood, such as Helix arbustorum, H. ericetorum, H. pulchella,
and its var. costata, Pupa, umbilicata, and various others.
The President and Professor Boulger made some remarks upon the
probable cause of white varieties in plants and animals.
Mr. R. M. Christy then read the following paper :—
On the Results of Interchanging the Eggs and Young of Different
Species of Birds.
It is well known that our common English Cuckoo habitually deposits
her egg, or eggs, in the nest of some small bird, and that the young
Cuckoo is reared by the rightful owners of this nest with apparent will-
ingness, and often to the exclusion of their own young. I do not ever
remember to have heard of a case in which the foster-parents have
refused to adopt the young creature thus unceremoniously thrust upon
them, or one in which they did not display as much solicitude for its
welfare as if it had been their own offspring. Although the nests of the
Titlark, the Pied Wagtail, and the Whitethroat are those most commonly
selected, yet there are perhaps as many as twenty or thirty species
altogether which are recorded to have had, at different times, the