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Page:Popular Science Monthly Volume 34.djvu/241

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eral introduction of vaccination the proportion has been reduced to less than one per cent.

The next most dreaded and at present the most fatal of these diseases is scarlet fever. In England, during the period already named, 6·7 per cent of all deaths under five were due to this cause; while an analysis of the registration reports of Massachusetts shows that two thirds of the deaths from this cause occur under five years of age.

Diphtheria is especially a filth-disease as well as a contagious disease; while the prevalence of each member of the class varies greatly in different localities and different years, being largely dependent upon certain unknown epidemic influences, which as yet have not been brought under the control of man.



THE organic relations or homologies of structure, showing connections of different beings with one another, which, in ignorance of their real bearing, were formerly made useful only for the classification of animals and plants into natural groups, have acquired a new significance since the doctrine of evolution has been brought to bear upon questions of origin and of the progress through time and space of living beings and their relations with those beings whose former existence is revealed by paleontology. The question whether there are evidences of affiliation of the former by the latter, of a direct relationship, has received attention from students; and, as one of the attempts to solve it, we may mention M. Gaudry's essay on "The Links in the Animal World," in which the development of the mammalia through geological times is investigated on the basis of the osseous frame. A difficulty that has never been surmounted besetting the study of the terrestrial mammalia with aërial respiration, arises out of their power of changing their place, which, while it is limited by geographical restrictions in the case of quadrupeds, is complete with birds. It is easy to conceive that overlappings and general irregularities have time and again been introduced into the combinations of groups of animals which any given country has successively contained. By that fact alone, the new-comers of each period, at points where we have not been able to observe their ancestors in the country of their origin, have an air of having risen suddenly and been preceded by nothing.

This is not so much the case in the vegetable kingdom, and is least so with its most eminent representative, the tree, particularly the forest tree or the tree that has become social. It is true that