. The causes and course of organic evolution; . even thoughLubbock half a century since wrote: If we judge animals bytheir intelligence as evinced in their actions, it is not the gorillaand the chimpanzee, but the bee, and above all the ant, whichapproach nearest to man {177: 65.) But, if the evidencesadvanced below prove anything, they clearly demonstrate thatsome genera or groups, owing to certain combined biologicalfactors, have run far ahead, in certain evolutionary character-istics, of the average members of their group. It is of primeimportance then to learn what these factors have been,
. The causes and course of organic evolution; . even thoughLubbock half a century since wrote: If we judge animals bytheir intelligence as evinced in their actions, it is not the gorillaand the chimpanzee, but the bee, and above all the ant, whichapproach nearest to man {177: 65.) But, if the evidencesadvanced below prove anything, they clearly demonstrate thatsome genera or groups, owing to certain combined biologicalfactors, have run far ahead, in certain evolutionary character-istics, of the average members of their group. It is of primeimportance then to learn what these factors have been, and howthey have operated. In the embryological development of every animal from thelower nemerteans upward, the first fundamental change notedin the multicellular embryo is the formation from the epi- Relation of Higher Animals to Man 547 dermis of a longitudinal tract that sinks inward to become thecentral nervous system. Next, three epidermic involutionsof tissue sink inward round the head region tojorm in almost Sacf^t \Suji)ef?nan. Croio. (eps Cuttlefcsh drac^iic^ /Tio/luscs Procb(irtApo/>o</s Fig. 27—Diagram to show comparative taxonomic and biologic relationof higher animals. invariable order of appearance (1) the olfactory tissue or spe-cial chemo tactic center, (2) the optic tissue, or special heliotacticcenter, (3) the auditory tissue or special tonotactic and geotac-tic tissue. Later other epidermal modifications, already re- 548 Causes and Course of Organic Evolution ferred to under Xemertinea (p. 420), Cyclostomata (p. 429),etc., arise over various parts of the skin, that form electrotactic,surface chemotactic, thermotactic, hydrotactic, mechanotactic,and thigmotactic tissues. These all correspond functionallyto certain tissues in higher plants, if we except the tonotactic,while in the animal, as in the plant, the response of the animalwhen these tissues are stimulated is a positive or negativechemotropic, heliotropic, paraheliotropic, thermotropic, geo-tr
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