Conditions governing deposition of eggs 269 
Kammerer has investigated the conditions for the laying of eggs in 
two forms of salamanders, e.g. Salamandra atra and S. maculosa. 
In both forms the eggs are fertilised in the body and begin to 
develop in the uterus. Since there is room only for a few larvae in 
the uterus, a large number of eggs perish and this number is the 
greater the longer the period of gestation. It thus happens that 
when the animals retain their eggs a long time, very few young ones 
are born; and these are in a rather advanced stage of development, 
owing to the long time which elapsed since they were fertilised. 
When the animal lays its eggs comparatively soon after copulation, 
many eggs (from 12 to 72) are produced and the larvae are of course 
in an early stage of development. In the early stage the larvae 
possess gills and can therefore live in water, while in later stages 
they have no gills and breathe through their lungs. Kammerer 
showed that both forms of Salamandra can be induced to lay their 
eggs early or late, according to the physical conditions surrounding 
them. If they are kept in water or in proximity to water and in 
a moist atmosphere they have a tendency to lay their eggs earlier 
and a comparatively high temperature enhances the tendency to 
shorten the period of gestation. If the salamanders are kept in 
comparative dryness they show a tendency to lay their eggs rather 
late and a low temperature enhances this tendency. 
Since Salamandra atra is found in rather dry alpine regions 
with a relatively low temperature and Salamandra maculosa in 
lower regions with plenty of water and a higher temperature, the 
fact that S. atra bears young which are already developed and 
beyond the stage of aquatic life, while S. maculosa bears young ones 
in an earlier stage, has been termed adaptation. Kammerer’s experi- 
ments, however, show that we are dealing with the direct effects 
of definite outside forces. While we may speak of adaptation when 
all or some of the variables which determine a reaction are un- 
known, it is obviously in the interest of further scientific progress 
to connect cause and effect directly whenever our knowledge allows 
us to do so. 
VII. ConcLupDING REMARKS. 
The discovery of De Vries, that new species may arise by muta- 
tion and the wide if not universal applicability of Mendel’s Law 
to phenomena of heredity, as shown especially by Bateson and his 
pupils, must, for the time being, if not permanently, serve as a basis 
for theories of evolution. These discoveries place before the experi- 
mental biologist the definite task of producing mutations by physico- 
chemical means. It is true that certain authors claim to have 
