38. In 1853, after reviewing the various attempts to express in a sentence what a volume could only approximately expound, I proposed the following: “Life is a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity.” This has been criticised by Mr. Herbert Spencer and by Dr. Lionel Beale, and if I had not withdrawn it before their criticisms appeared, I should certainly have modified and enlarged it afterwards. I mention it, however, because it is an approach to a more satisfactory formula in so far as it specifies two cardinal characteristics distinguishing organisms from all anorganisms, namely, the incessant evolution through definite stages, and the preservation of specific integrity throughout the changes; not only the organism as a whole is preserved amidst incessant molecular change, but each tissue lives only so long as the reciprocal molecular composition and decomposition persist. On both of these points I shall have to speak hereafter. The definition, however, is not only defective in its restriction to the molecular changes of Nutrition, taking no account of the Properties and Functions of the organism; but defective also in giving no expression to equally important relations of the organism to the medium. 39. This last point is distinctly expressed in Mr. Spencer’s definition: “Life is the continuous adjustment of internal relations to external relations.” Considered as a formula of the most general significance, embracing therefore what is common to all orders of vital phenomena, this is the best yet proposed.[9] If I propose another it will not be to displace but to run alongside with Mr. Spencer’s; and this only for more ready convenience. Before doing so I must say a few words by way of clearing the ground.