218 
Heredity. 
nutritive value, and requires no great effort to obtain its food, 
which lies heaped up around it. The larva of the blow-fly becomes 
a pupa in eight to ten days, although it moves actively in boring 
its way under the skin and into the tissues of the dead animals upon 
which it lives. The life of the leaf-eating caterpillars of butterflies 
and moths lasts for six weeks or longer, corresponding with the 
lower nutritive value of their food and the greater expenditure of 
muscular energy in obtaining it. Caterpillars which live upon wood 
have a larval life of two or three years. There is, however, no 
essential relation between duration of life in the larva and in the 
mature insect— it does not follow, for example, that, if the larva is 
long-lived, the mature insect is necessarily short-lived. The life of 
the mature insect (the imago) is generally very short, and ends 
with the close of the period of reproduction, which is itself extremely 
short. 
"The shortest life i9 found in theimagos of certain May-flies, which only 
live four to five hours. They emerge from the pupa-case towards the even- 
ing, and as soon as their wings have hardened they begin to fly, and pair 
with one another. Then they hover over the water; their eggs are extruded 
all at once, and death follows almost immediately." 
A principle of great importance is embodied in the following 
words : — 
" Insects belong to the number of those animals which, even in their 
mature state, are very liable to be destroyed by others which are dependent 
upon them for food ; but they are, at the s-»me time, among the most fertile of 
animals, and often produce au astonishing number of eggs in a very short 
time. And no better arrangement for the maintenance of the species under 
such circumstances can be imagined than that supplied by diminishing the 
duration of life, and simultaneously increasing the rapidity of reproduction." 
The phenomenon of parthenogenesis (virgin birth), as witnessed 
in aphides and other insects, is treated at considerable length ; and it 
is, indeed, Scarcely possible to turn to any page of this fascinat- 
ing work without finding something to arrest the eye and to exercise 
the mind. 
The great interest which Weismann's essays have excited 
throughout the scientific world affords high testimony to their 
inherent merits. The problems which he has formulated are»of pro- 
found significance, and it is not out of place to suggest that stock- 
breeders possess many and exceptional advantages for making solid 
additions to the storehouse of facts in this department of scientific 
inquiry. Such excellent work has been done in this country in the 
improvement of the breeds of farm-animals that it seems deplorable 
that the men, whose keen eye and subtle touch have achieved results 
of world-wide fame, should have gone to the grave and left no written 
record behind them. If any breeder would accurately and honestly 
record all his successes and all his failures — especially the failures — 
in the mating of selected animals, be would in time compile a history 
of inestimable value. The work would need to be done with the 
most scrupulous exactitude, and the most rigid control of every 
observation. Hut, besides adding to the sum-total of useful know 
