others, but many, especially Darwin himself in his later years, had nevertheless been disposed to depreciate the significance of such facts. They consequently then lapsed into general disparagement. Upon more careful inquiry the abundance of such phenomena proved to be far greater than was currently supposed, and a discussion of their nature brought into prominence a consideration of greater weight, namely that the differences by which these definite or discontinuous variations are constituted again and again approximate to and are comparable with the class of differences by which species are distinguished from each other. The interest of such observations could no longer be denied. The more they were examined the more apparent it became that by means of the facts of variation a new light was obtained on the physiological composition and capabilities of living things. Genetics thus cease to be merely a method of investigating theories of evolution or of the origin of species but provide a novel and hitherto untried instrument by which the nature of the living organism may be explored. Just as in the study of non-living matter science began by regarding the external properties of weight, opacity, colour, hardness, mode of occurrence, etc., noting only such evidences of chemical attributes and powers as chance spontaneously revealed; and much later proceeded to the discovery that these casual manifestations of chemical properties, rightly interpreted, afford a key to the intrinsic nature of the diversity of matter, so in biology, having examined those features of living things which ordinary observations can perceive, we come at last to realize that when studied for their own sake the properties of living organisms in respect of heredity and variation are indications of their inner nature and provide evidences of that nature which can be obtained from no other source. While such ideas were gradually forming in our minds, came the rediscovery of Mendel's work. Investigations which before had only been imagined as desirable now became easy to pursue, and questions as to the genetic inter-relations and compositions of varieties can now be definitely answered. Without prejudice to what the future may disclose whether by way of limitation or extension of Mendelian method, it can be declared with confidence and certainty that we have now the means of beginning an analysis of living organisms, and distinguishing many of the units or factors which essentially determine and cause the development of their several attributes.