358 
PEOFERSOP. K. PE APRON AND OTHERR ON 
and the value for longevity in man much below the true fraternal correlation. In 
the former case, the mothers were few in number, in the latter the non-selective 
death-rate reduces very considerably the intensity of collateral inheritance. Both 
longevity and tem|)er are included in this table for the same reason as Nigella 
Hispayiica and woodruff in the first table. T would not run the risk of any apparent 
selection to reduce either homotypic or fraternal correlation to a closer range of values. 
The mean of this second table gives the value '4479 for fraternal correlation. Now I 
do not propose to lay great stress on what at first sight might look like a most con¬ 
clusive equality lietween the mean values of homotypic and fraternal correlations,— 
within tlie limits of the probable errors '4479 and •4570 are indeed equal. I am quite 
aware that a few further series added to either the homotypic or fraternal results 
might modify to some extent this equality. But what I would ask the reader to do 
is to examine the two tables side by side, to note how the first and last several 
results of Ijoth may fairly be held to be suliject to quite definite modifying factors, 
and then to consider whether there is not very substantial evidence gathered from a 
fairly wide range of characters in nearly as wide a range of species to show that both 
homotypic and fraternal correlation fluctuate about a mean value between '4 and ‘5. 
I will not venture to assert that either are absolutely constant, but I do realise 
that it is extremely difficult with the complex system of factors influencing living 
forms to reduce our conditions to that theoretically perfect state in which we shall 
measure solely the factor we are investigating. If the intensity of homot3q30sis were 
exactly "45, I should be inclined to distrust any long series of results, one and all of 
which gave the answer ‘45 exactly. Tliere are so many other disturbing factors which 
only those who have endeavoured to collect series of this kind will fully appreciate. 
In tlie first place, the theoretical conception of undifferentiated like organs is veiy 
hard to realise practically ; position of the organ on the branch or of the branch on a 
plant, however careful be the collector, may really have introduced differentiation, and 
so weakened the apparent homotyj^osis. Secondly, the environmental factor comes 
into play. It is difficult to obtain a hundred individuals with like environment; soil, 
position with regard to other growths, sunlight, insect life, &c., may differ in a manner 
that the collector cannot appreciate. Unlike environment may produce a fictitious 
likeness in the organs of the same individual when we pass from one individual to a 
second. The fact tliat half our series grew in one field, the other half in another, 
that part came' from one side of a road, part from another, may introduce an uiq)er- 
ceived heterogeneity whicli increases the apparent homotyposis. Thirdly, the 
difficulty of ensuring that all individuals are of the same age or in the same stage 
of development, is very great. Tlie leaves of an old tree may have a fictitious 
likeness when compared with those of a young tree ; we may gather organs from one 
individual when it is in a stage of development, which would onlv be reached in 
another individual some days or weeks later. These and other factors may perhaps 
be to some extent eliminated—far better of course by the trained botanist than by 
