226 The Mechanistic Conception of Life 



seventy-two) 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 Ivmgs. 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 drjmess they show a 

 tendency to lay their eggs rather late and a low temperature 

 enhances this tendency. 



Since Salamandra atra is foimd 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 

 yoimg ones in an earlier stage, has been termed adaptation. 

 Kammerer's experiments, 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 imknown, 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. concluding eemarks 



The discovery of DeVries, that new species may arise by 

 mutation 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 experimental biologist the definite task of pro- 

 ducing mutations by physico-chemical means. It is true that 



