Among plants we find thousands of instances of such reactions of the organism to external stimulus--reactions which are not of a primary nature, that is, are not the inevitable consequences of the plant's constitution, but which depend upon adaptations of the special constitution of a species or group of species to the specific conditions of its life. To this category belong all the phenomena of heliotropism, geotropism, and chemotropism, which have been discovered by the numerous and excellent observations of the plant physiologists. That all these are adaptations and secondary reactions to stimuli is proved by the fact that the same stimuli affect the homologous parts of different species in very different, and often in opposite ways. For instance, while the green shoots of most plants turn towards the light, being positively heliotropic, the climbing shoots of the ivy and the gourd are negatively heliotropic, which is an adaptation to climbing. In this case the reason of the difference in the mode of reaction must lie in the difference of constitution of the cellular substance of the shoot, and since this may differentiate so very diversely in its relation to light, the power of reaction which plant substance in general has to light must not be regarded as a primary character, like the specific gravity of a metal or the chemical affinities of oxygen and hydrogen, but as adaptations of the living and varying substance to the special conditions of life. And the origin of these adaptations must depend upon processes of selection, and on these alone. This is just the difference between living and non-living matter,--that the former is variable to a high degree, the latter is not; it is the fundamental difference upon which the whole possibility of the origin of an animate world depends. Among animals also we must distinguish between the direct effects of external influence to which the organism is not already adapted, and those reactions which imply a previously established adjustment to the stimulus. That is, we must distinguish between primary and secondary reactions. For instance, Herbst made artificial sea-water in which the sodium was partially replaced by lithium, and the eggs of sea-urchins developed in this artificial sea-water into very divergent larvæ of peculiar structure. We have here a primary reaction of the organism to changed conditions of life--not an adaptation, not a prepared reaction. Accordingly these 'lithium larvæ' eventually perished.