648 AMERICAN ANTHROPOLOGIST [n. s., i, 1899
convenient to assume that the course of mental development is approximately uniform (or about as nearly similar as are environ- • mental conditions) in each separate or independent group of men — this assumption, which is rapidly crystallizing in the minds of anthropologists, being but a corollary of the primary postulate on which all science rests, namely, that knowledge grows by natural means.
The mere recognition of these principles renders it clear that the particular growth-stage of any intellectual stock (or people) may be defined with approximate accuracy by gauging the mentality of other peoples developed to corresponding stage, just as the history of theaged sequoia grove of prehistoric birth may be read in terms of younger groves on neighboring ranges ; for the towering forests of the big-tree species and the upshooting forests of human ideas may well be likened in individual and collective growth, save that the vegetal species is decadent and shrinking into separate groves in scattered holts, while the mental growth is luxuriant and spreading exuberantly from province to province throughout the lands of the earth. The interpretation in terms of growth-stages is established by conformity with nat- ural law ; did the forest receive extra-natural impulse at any stage, or did knowledge arise otherwise than through interaction with nature, the interpretation would fail ; but in the absence of evidence against the uniformity of nature, the equivalence of corresponding stages must be recognized alike for the figurative forests of ideas and the material forests of wood and leafage.
The acceptation of these cardinal principles affords a means of tracing the unrecorded history of Aryan culture, and of inter- preting the meager records of Arabia's mathematical pioneering in terms of the culture of other peoples still below, or just rising upon, the plane marked by the birth of writing. Especially useful for comparison are various practically independent Amer- indian peoples, some low in prescriptorial culture, others trem- bling on the verge of definite graphic art, and still others within