XC1V 
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 co mm on, 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 uvibilicata, and various others. 
The President and Professor Boulger made some remarks upon the 
probable cause of white varieties in plants and animals. 
Mr. E. 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 
