Summary and General Conclusions 461 
this idea of many individuals varying at the same time, and 
in the same direction, at once involves the conception that 
evolution moves forward by some force residing in the organ- 
ism, driving it forwards or backwards. Instability comes, 
perhaps, nearer to expressing this idea than any other term, 
and yet to evolve from a protozoan to a man implies the idea 
of something more than simple unstableness. 
The idea that Weismann has touched upon in this connec- 
tion, namely, that the survival of a given form determines the 
future course of evolution for that form, is very plausible, and 
also fits in well with the results of our experience in the field 
of the inheritance of variations. We see new variations or mu- 
tations departing in some or in many characters from the 
original type, apparently by new combinations or perturba- 
tions of those already present. We never expect to see a 
bird emerge from the egg of an alligator. Thus it appears 
that by the survival of certain forms the future course of 
evolution is determined in so far as the new types of muta- 
tion are thereby limited. Weismann means, however, that 
in this way new plus or minus steps will be indefinitely deter- 
mined amongst the new fluctuating variations, but this state- 
ment is contradicted by our experience of the results of 
artificial selection. The upper limit does not keep on pushing 
out indefinitely in the direction determined by the first selec- 
tion, but is soon brought to a standstill. So that, as far as 
Weismann’s hypothesis is concerned, the idea appears to have 
no special value. On the other hand, this idea may be fruitful 
if applied to mutations, but here unfortunately we have not 
sufficient experience to guide us, and we do not know defi- 
nitely whether a new character that appears as a mutation 
will be more likely, in subsequent mutations, to go on increas- 
ing in some of the descendants. Thus, while the mutation 
theory must assume that some new characters will go on 
heaping up, we lack the experimental evidence to show that 
this really occurs. It would be also equally important to 
