Oswald H. Latter 
165 
callipers reading to millimetres, the decimal parts of a millimetre being estimated 
by eye aided with a strong lens. The dimensions measured are greatest length 
and greatest breadth. 
It has been established by many observers that the female Cuckoo lays her 
egg upon the ground and then taking it in her beak puts it into the nest of 
the foster-parents of her offspring. An explanation is needed of the success which 
attends this imposition. Are the foster-parents deceived either by similarity 
of colouring or of size into fancying the Cuckoo's egg to be one of their own ? 
or are they indifferent to these qualities ? or are some small birds more expert 
than others in detecting fraud ? 
The theory which finds more favour than others is that put forward by 
Prof A. Newton {Dictionary of Birds, p. 123); who, after mentioning the history 
of speculation on the matter, writes as follows : — " Everyone who has sufficiently 
studied the habits of animals will admit the tendency of some of tliese habits 
to become hereditary. That there is a reasonable probability of each Cuckow 
most commonly putting her eggs in the nest of the same species of bird, and 
of this habit being transmitted to her posterity, does not seem to be a very violent 
supposition. Without attributing any wonderful sagacity to her, it does not seem 
unlikely that the Cuckow which had once successfully foisted her egg on a 
Reed-wren or a Titlark should again seek for another Reed-wren's or another 
Titlark's nest (as the case may be) and that she should continue her practice 
from one season to another Such a habit could hardly fail to become 
hereditary, so that the daughter of a Cuckow which always put her egg into 
a Reed-wren's, Titlark's, or Wagtail's nest, would do as did her mother 
and it can hardly be questioned that the eggs of the daughter would more or 
less resemble those of her mother. Hence the supposition may be fairly regarded 
that the habit of laying a particular style of egg is also likely to become 
hereditary The particular ' gens ' of Cuckow which inherited and trans- 
mitted the habit of depositing in the nest of any particular species of bird eggs 
having more or less resemblance to the eggs of that species would prosper most 
in those members of the ' gens ' where the likeness was strongest, and the other 
members would (caeteris paribus) in time be eliminated The operation of 
this kind of natural selection would be most needed in those cases where the 
species are not easily duped — that is in those cases which occur the least 
frequently. Here it is we find it, for observation shows that eggs of the Cuckow 
deposited in the nests of the Red-backed Shrike, of the Bunting, of the Red 
Start and of the Icterine Warbler approximate in their colouring to eggs of these 
species — species in whose nests the Cuckow rarely (in comparison with others) 
deposits eggs." 
I must confess that I approached this investigation with decided scepticism as 
to the validity of Prof Newton's theory. It is very doubtful whether the Cuckoo 
is aware that she has "successfully foisted her egg on a Reed-wren" or on any 
other bird : so far as is known she takes no further interest in the egg — it may 
