[DV] Darwin, "Animals and Plants under Domestication," vol. ii. p. 70. [DW] "Organic Evolution," Mr. Cunningham's translation, p. 76. [DX] Darwin, "Animals and Plants under Domestication," vol. i. p. 104. [DY] Similarly, from a chance sport of a one-eared rabbit, Anderson formed a breed which steadily produced one-eared rabbits ("Animals and Plants under Domestication," vol. i. p. 456). This is an example of asymmetrical variation. Variations are generally, but not always, symmetrical. Superficial colour-variations are sometimes asymmetrical. Gasteropod molluscs are nearly always asymmetrically developed. Among insects, _Anisognathus_ affords an example of the asymmetrical development of the mandible. Our right-handedness is a mark of asymmetry. [DZ] "Natural Inheritance," p. 32. [EA] See "Animals and Plants under Domestication," vol. ii. p. 40, from which illustrations are taken. [EB] "Evolution and Disease," p. 169. [EC] "Animals and Plants under Domestication," vol. ii. p. 8. [ED] "Darwinism," p. 107. [EE] Darwin, "Animals and Plants under Domestication," vol. ii. pp. 17, 18. [EF] "Animals and Plants under Domestication," vol. ii. p. 201. [EG] Ibid. p. 282. The phenomena of the seasonal dimorphism of butterflies and moths show that changes of temperature (and perhaps moisture, etc.) determine very striking differences in these insects. [EH] "Animals and Plants under Domestication," vol. ii. p. 244. [EI] "Darwinism," p. 293. [EJ] "Evolution of Sex," p. 22. [EK] "Incidental Observations in Pedigree Moth-breeding," F. Merrifield. Transactions Entomological Society, 1889, pt. i. p. 79, _et seq._ [EL] _Nature_, vol. xli. p. 393. [EM] See Professor Meldola's edition of Professor Weismann's "Studies in the Theory of Descent," and Mr. Cunningham's translation of Professor Eimer's "Organic Evolution." [EN] See Darwin, "Animals and Plants under Domestication," vol. ii. p. 252. CHAPTER VII. THE SENSES OF ANIMALS. It is part of the essential nature of an animal to be receptive and responsive. The forces of nature rain their influence upon it; and it reacts to their influence in certain special ways. Other organisms surround it, compete with it, contend with it, strive to prey upon it, and occasionally lend it their aid. It has to adjust itself to this complex environment.