PROTECTIVE RESEMBLANCE 
21 
passed before an abundance of aberrations of many kinds 
again made its appearance in 1918, when the occurrence was 
similar to that in the season of 1881 ; thus a period of thirty- 
seven years intervened between these two remarkable seasons 
for aberrations. In both years the phenomenon was precisely 
similar as regards the abundance of insects and their aberra¬ 
tion. Both the previous winters were much of the same 
severity. On January 18th, 1881, the great blizzard swept 
the whole of Britain, with snow-drifts obliterating the country¬ 
side ; the following July tropical heat prevailed, with almost 
continual sunshine. The winter 1917-18 was likewise one of 
arctic severity, followed by blizzards of snow in April and 
almost tropical spells of heat in May and June, with stormy 
weather during July. If the prevalence of aberration was 
occasioned by climatic influences, the effect would have taken 
place during the spring and early summer ; therefore the 
weather of July, the month for the emergence of the insects, 
would be ineffectual to their occurrence, as the influence would 
take place in their earlier stages. 
Individual aberration of abnormal colouring appears from 
time to time in animals of all kinds. The more general type 
is towards albinism, resulting in partial or complete albinistic 
examples, caused by the absence of the dark pigment in the 
general structure. On the other hand, melanism is produced 
by an excessive supply of dark pigment either scattered in 
small deposits or spread over the body of an individual, thus 
producing different degrees of melanism. Probably these 
phases of aberration at the present time are reversions to 
ancestral forms. 
A remarkable and interesting fact is the recurrence of 
certain phases of aberrant specimens totally distinct from 
the normal species both in pattern and colouring of the 
markings, but every now and again turning up as single 
examples in widely different localities and in different years. 
7 he cause of these recurrent aberrant individuals cannot be 
attributed to climatic conditions, locality, or to the nature 
of the food plant of the caterpillars. This I have proved 
over and over again by rearing specimens from eggs laid by 
normal parents. Take, for instance, the Small Tortoiseshell, 
which is capable of laying about 1,000 eggs in a batch. Now, 
