Page:Popular Science Monthly Volume 32.djvu/119

Rh uses the 'theory of descent' as a working hypothesis, without which no one studying any group of animals in the period of its rise and most rapid evolution can expect to do otherwise than stumble and wander astray. To refuse it is to merit failure."

Professor J. S. Kingsley, in his study of Limulus, regards it as an Arachnid, but states that its ancestors take us back to a time when the distinctions between the Crustacea and Arachnida were far less marked than now.

Dr. A. S. Packard, in a paper on the "Genealogy of the Insects," shows by means of a "genealogical tree" the descent of the class from the Thysanura, with some hypothetical creature not unlike Scolopendrella, as the probable stem-form of the hexapods. It is through the resemblance the larvæ of the different orders of insects bear to various members of the Thysanura that this scheme is justified. It may not be out of place to say here that the use of the "genealogical tree," in suggesting the probable line of descent of various allied groups, has been severely condemned by some as leading to no practical good in classification. It seems to me, however, the only clear scheme for the proper working out of the ascertained or hypothetical relationsliips of animals; it is thought-exciting, its very attitude provokes studious inquiry and suggestive inferences. It may be called the modern tree of knowledge.

The modern genealogical tree as used by the biological student (and as well by the ethnologist, philologist, and others) is a graphic diagram of the relationships between groups as understood by the projector, and, as such, is a most commendable and useful method with which to illustrate his meaning. With additional knowledge one can see at a glance the points that need strengthening, and he can pare, prune, or even graft new fruits on the old stock, or, if it is rotten at the trunk, cut it down altogether. These trees have always been in vogue with the older naturalists, only, in the old style of arboriculture, the trunk was always kept stiffly vertical, while the branches were bent down and trained horizontally, being flimsily attached to the main stem by printers' devices of long and short brackets. In this attitude it reminded one of the dwarfed and deformed trees of the Chinese, and very properly typified the dwarfed and deformed way of looking at classification.

Never was the provisional use of a genealogical tree more completely justified than m a memoir by Dr. Alexander Agassiz on the "Connection between Cretaceous and Echinid Faunæ." He certainly speaks in uncertain terms when, in considering the Spatangoids of the chalk, he says, "They lead us directly through the Palæostominæ and the Collyritidæ to the Ananchytidæ which have persisted to the present day," and other relationships of the same nature are