Ernest Warren 
133 
The normal duration of life of my aphides was about three weeks to a month. 
If during the period of active growth the environment is unfavourable the indi- 
viduals become permanently small and will never attain to a considerable size. 
They apparently become mature and will live three weeks or more, but will 
ultimately die undersized however favourable the environment may afterwards 
become. 
On account of this great sensitiveness of the organism to its environment it can 
be seen that to measure the correct correlation between offspring and parent (at 
least with respect to absolute dimensions) all the broods of the generation ought 
to experience an identical environment. The fact that the conditions of life were 
progressively less ftxvourable, more especially with the grandchildren, would not 
necessarily alter the correlation if all the broods were aifected alike. In practice 
it is not wholly possible to provide an identical environment for all the broods ; 
temperature, humidity of the air, illumination, etc., were nearly the same for all 
the broods, but tlie leaves of a plant differ from one another very considerably in 
succulence, the amount of decomposition of the chlorophyll, etc. To eliminate as 
much as possible these differences I always endeavoured to select similar leaves for 
the broods to live on. 
The differences which were observed among the individuals of the generation, 
when growth had practically ceased, must be ascribed to the laws of variation and 
inheritance, and it is to these differences that I shall return when discussing the 
variability and the intensity of inheritance in parthenogenetically produced 
offspring. 
(4) Fertility. 
The birth of young by a mature female is not a continuous process, a brood of 
about 6 — 12 offspring is produced in two or three days and then there is a pause 
before others are born. It is probable that the number of individuals in a brood 
TABLE II. 
No. ill Brooil 
5 
6' 
8 
.9 
10 
12 
IS 
u 
ir, 
No. of 
Families 
Total No. 
of Offspring 
Frequency of 
newly born 
2 
8 
15 
7 
7 
9 
2 
6 
1 
2 
1 
60 
522 
Frequency of 
adults . . . 
3 
7 
8 
19 
7 
5 
5 
1 
2 
1 
2 
00 
455 
No. of winged 
females ... 
2 
3 
1 
_ 
3 
9 
in all 
bears a definite relation to the fertility of the mother which produced it. In the 
second row of Table II, is given the frequency of the number of individuals in the 
newly born families and in the third row the frequency of the adult broods. If 
