41 days after the summer solstice for the North Polar. 61 days after the summer solstice for the Arctic. 95 days after the summer solstice for the Sub-Arctic. We now pass to the other curves, those that were unaffected by cold. Though in these the minima themselves show the law of latitudinal progression, the wavelike character of the advance is even better disclosed by the curves. As the eye follows them down the page, the advance of the wave to the right is plainly apparent. The slope of the wave is much the same for all, implying that a like force was at work successively down the latitudes. It will be noticed next that in all the mean cartouches the gradient is greater after the minimum than before it. The curves fall gently to their lowest points and rise more steeply from them. Such profile indicates that the effects of a previous force were slowly dying out down to the minimum and that then an impulse started in to act afresh. This explains the attitude of the canals that died out. In them the effect of the old force shows as in the others, but no impulse came in their case to resuscitation. It seems possible to trace this force to an origin at the south. For beginning with the north sub-tropic zone the gradient on the left shows less and less steep southward to the south sub-tropic zone. Such a dying-down swell is what should be looked for in an impulse which had travelled from the south northward, since the wave would affect the more northern zones last, and less of a calm period would intervene between the two impulses from opposite poles. The cartouches, then, state that the canals began to develop after the greatest melting of the polar cap had occurred; that this development proceeded down the latitudes to the equator, and then not stopping there advanced up the latitudes of the other hemisphere. In the next place they show that in the arctic region the development was arrested and devolution or decay set in as it began to get cold there, the most northern canals being affected first. Finally, that a similar wave of evolution had occurred from the opposite pole some time before and had then passed away. And this evidence of the cartouches is direct, and independent of any theory. CHAPTER XXIV CANAL DEVELOPMENT _Individually Instanced_ As an interesting instance of the law of development we may take the career of the Brontes during this same Martian year; the Brontes witnessing individually to the same evolutionary process that the canals collectively exhibit.