A change arising from purely internal causes seems to me above all quite untenable, because I cannot imagine how the same material substratum of physical constitution of a species can be transferred to the succeeding generation as two opposing tendencies. Yet this must be the case if the direction of development transferred by heredity is to be regarded as the ultimate ground both of the similarity and dissimilarity to the ancestors. All changes, from the least to the greatest, appear to me to depend ultimately only on external influences; they are the response of the organism to external inciting causes. It is evident that this response must be different when a physical constitution of a different nature is affected by the same inciting cause, and upon this, according to my view, depends the great importance of these constitutional differences. If, under “heredity,” we comprise the totality of inheritance--that is to say, the physical constitution of a species at any time, and therefore the restricted and, in the foregoing sense, pre-determined power of variation, whilst under “adaptation” we comprehend the direct and indirect response of this physical constitution to the changes in the conditions of life, I can agree with Haeckel’s mode of expression, and with him trace the transformation of species to the two factors of heredity and adaptation. APPENDIX I. EXPERIMENTS. EXPERIMENTS WITH ARASCHNIA LEVANA. 1. Bred from eggs laid by a female of the winter form on 12th-15th May, 1868, in a breeding-cage. The caterpillars emerged on 20th-22nd May, and pupated on 7th-9th June. The pupæ, kept at the ordinary temperature, produced:-- On the 19th of June 4 butterflies. ” 20th ” 5 ” ” 21st ” 10 ” ” 22nd ” 9 ” ” 23rd ” 7 ” ” 25th ” 13 ” -- Total 48 ” All these butterflies were of the _Prorsa_ type, 3 females having a considerable amount of yellow, but none with so much as figs. 3, 4, 7, 8, or 9. Pl. I. 2. August 12th, 1868, found larvæ of the third generation, which pupated at the beginning of September, and were kept in a room not warmed. In September three butterflies emerged in the _Prorsa_ form, the remainder hibernating and producing, after being placed in a heated room at the end of February, from the 1st to the 17th of March, 1869, more butterflies, all of the _Levana_ form. 3. Larvæ found on the 17th June, 1869, were sorted according to colour; the yellow ones, with light brown spines, produced, at the ordinary temperature, on 8th-12th July, 13 butterflies, 12 of which showed the ordinary _Prorsa_ type, and one, a male, possessing more yellow than fig. 3, Pl. I., must be considered as a _Porima_ type.