If you deem the 'chief difference' between Darwin's and Weismann's theory of heredity to be 'that the one implies _material particles_ and the other only _physical and chemical constitution_,' then, it seems to me, Weismann's theory will become identical with Herbert Spencer's--seeing that this is virtually the only respect in which Spencer's differs from Darwin's. But I think there is another and a much more important respect in which W.'s theory differs from both these predecessors. However, to proceed to the next point, I agree with you, that the sole object of the Sphex stinging the larvæ is _now_ to cause them to 'keep,' and that natural selection must have worked upon this for perfecting the instinct. But the point is, what was the origin of the selective stinging? If merely chance congenital variations, would unity to billions express the chances against their ever arising? Get some mathematician to calculate--giving as data superficial area of caterpillar on the one hand and that of _nine_ ganglia on the other. Even neglecting the consideration that the variation must occur many times to give unaided natural selection a chance to fix it as an instinct, the chances against its occurring only once would be represented by the following series, where _x_ is the superficial area of the caterpillar _minus_ that of eight ganglia, and unity is superficial area of one ganglia: 1/x × 1/x × 1/x × 1/x × 1/x × 1/x × 1/x × 1/x × 1/x If, as I suppose, _x_ may here be taken as = 100,000, the chances against the variation occurring once would be written in figures expressing unity to one thousand million billion trillions. Of course I do not rely on calculations of this kind for giving anything like accurate results (mathematics in biology always seems to me like a scalpel in a carpenter's shop), but it makes no difference how far one cuts down such figures as these. Therefore, if Lamarck won't satisfy such facts, neither do I think that Darwin _minus_ Lamarck can do so. We must wait for the next man. I will send you 'Perrier' on my return to town next month. Lord Morton's experience is so universally that of all breeders of live stock, that I never knew anybody ever doubted it. But, if they do, there is no reason why they should not satisfy themselves on the point. For my part I do not feel that the fact requires any corroboration as regards mammals, though I have some experiments going on with birds. Lastly, the apparently analogous cases in plants are still worse for Weismann's theory, and they stand on the best authorities.