Exceptions to this rule, as exemplified by the cats, are due to special adaptive causes.
The stimulus of water on the breast may be regarded as a sensory presentation which is followed by a definite and adaptive application of behaviour.
The rapid variation of certain groups of animals or the acceleration of certain organs is also not evidence of the sudden appearance of new adaptive characters.
In endeavouring to trace the causation of adaptation, it is obvious that it must be due quite as much to properties inherent in the plant as to the action of external conditions; the plant must possess adaptive capacity.
Of these two kinds of genetic and adaptive resemblance, homogeny is the warp composed of the vertical, hereditary strands, which connect animals with their ancestors and their successors, while analogy is the woof, composed of the horizontal strands which tie animals together by their superficial resemblances.