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In designing boilers of this style it is necessary to guard against having the uptake at the upper end of the tubes too large, for if sufficiently large to allow downward currents therein, the whole effect of the rising column in increasing the circulation in the tubes is nullified (Fig. 7). This will readily be seen if we consider the uptake very large when the only head producing circulation in the tubes will be that due to the inclination of each tube taken by itself. This objection is only overcome when the uptake is so small as to be entirely filled with the ascending current of mingled steam and water. It is also necessary that this uptake should be practically direct, and it should not be composed of frequent enlargements and contractions. Take, for instance, a boiler well known in Europe, copied and sold here under another name. It is made up of inclined tubes secured by pairs into boxes at the ends, which boxes are made to communicate with each other by return bends opposite the ends of the tubes. These boxes and return bends form an irregular uptake, whereby the steam is expected to rise to a reservoir above. You will notice (Fig. 8) that the upward current of steam and water in the return bend meets and directly antagonizes the upward current in the adjoining tube. Only one result can follow. If their velocities are equal, the momentum of both will be neutralized and all circulation stopped, or, if one be stronger, it will cause a back flow in the other by the amount of difference in force, with practically the same result.

It is obvious that even the problem of _coadaptation in sterile animals_ can thus be satisfactorily explained. If the determinants are oscillating upwards and downwards in continual fluctuation, and varying more pronouncedly now in one direction now in the other, useful variations of every determinant will continually present themselves anew, and may, in the course of generations, be combined with one another in various ways. But there is one character of the determinants that greatly facilitates this complex process of selection, that, after a certain limit has been reached, they go on varying in the same direction. From this it follows that development along a path once struck out may proceed without the continual intervention of personal selection. This factor only operates, so to speak, at the beginning, when it selects the determinants which are varying in the right direction, and again at the end, when it is necessary to put a check upon further variation. In addition to this, enormously long periods have been available for all these adaptations, as the very gradual transition stages between females and workers in many species plainly show, and thus this process of transformation loses the marvellous and mysterious character that seemed at the first glance to invest it, and takes rank, without any straining, among the other processes of selection. It seems to me that, from the facts that sterile animal forms can adapt themselves to new vital functions, their superfluous parts degenerate, and the parts more used adapt themselves in an ascending direction, those less used in a descending direction, we must draw the conclusion that harmonious adaptation here comes about _without the cooeperation of the Lamarckian principle_. This conclusion once established, however, we have no reason to refer the thousands of cases of harmonious adaptation, which occur in exactly the same way among other animals or plants, to a principle, the _active intervention of which in the transformation of species is nowhere proved. We do not require it to explain the facts, and therefore we must not assume it._

In my dream I saw a fertile plain, rich with the hues of Autumn. Tranquil it was and warm. Men and women, children, and the beasts worked and played and wandered there in peace. Under the blue sky and the white clouds low-hanging, great trees shaded the fields; and from all the land there arose a murmur as from bees clustering on the rose-colored blossoms of tall clover. And, in my dream, I roamed, looking into every face, the faces of prosperity, broad and well favored--of people living in a land of plenty, of people drinking of the joy of life, caring nothing for the morrow. But I could not see their eyes, that seemed ever cast down, gazing at the ground, watching the progress of their feet over the rich grass and the golden leaves already fallen from the trees. The longer I walked among them the more I wondered that never was I suffered to see the eyes of any, not even of the little children, not even of the beasts. It was as if ordinance had gone forth that their eyes should be banded with invisibility.

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