At first, for many reasons, Weismann’s theories did not meet with general acceptance, but during recent years there has been a marked return to many of his positions, mainly as the result of further cytological discoveries, and of the formulation of Mendel’s Law and of De Vries’s mutation theory.  The first-named law was propounded by Gregor Mendel on the basis of extensive experiments upon plants conducted during many years, 1860 and later, in the obscurity of his monastery garden at Altbruenn, in Austria.  It was rescued from oblivion by De Vries, who found it buried in a mass of literature and brought it to light when he published his renowned Mutation Theory in 1901.  Mendelian phenomena of inheritance, confirmed and extended by numerous workers with plants and animals, prove that in many cases portions of the streams of germ plasm that combine to form the hereditary content of organisms may retain their individuality during embryonic and later development, and that they may emerge in their original purity when the germ-cells destined to form a later generation undergo the preparatory processes of maturation.  They demonstrate also the apparent chance nature of the phenomena of inheritance.  To my mind the most striking and significant result in this field is the demonstration that a particular chromosome or chromatin mass determines a particular character of an adult organism, which is quite a different matter from the reference of all the hereditary characters to the chromatin as a whole.  Wilson and others have brought forward convincing proof that the complex character of sex in insects actually resides in or is determined by particular and definite masses of this wonderful physical basis of inheritance.

Mendel’s principles also account in the most remarkable way for many previously obscure phenomena, like reversion, or a case where a child resembles its grandparent more than it does either of its parents; such phenomena are due, so to speak, to the rise to the surface of a hidden stream of germ plasm that had flowed for one or many generations beneath its accompanying currents.  I believe that the law is replacing more and more the laws of Galton and Pearson, formulated as statistical summaries of certain phenomena of human inheritance taken en masse.  According to Galton’s celebrated law of ancestral inheritance, the qualities of any organism are determined to the extent of a certain fraction by its two parents taken together as a “mid-parent,” that a smaller definite fraction is contributed by the grandparents taken together as a mid-grandparent, and so on to earlier generations.  But Mendel’s Law has far greater definiteness, it explains more accurately the cases of alternative inheritance, and it may be shown to hold for blended and mosaic inheritance as well.