Lecture XXIX. 261 



acquired characters. 1 Thus so far as accurate observation only is 

 relied on, no character, be it a deformity or an " acquirement," 

 which is directly derived from the action of the environment, has 

 been shown to be transmitted from the parent to its offspring. 

 Hence it is generally believed that the surroundings are without 

 direct influence on the germ-plasm or hereditaiy substance. 



The study of heredity has attracted attention from the earliest 

 times not only owing to its great intrinsic interest as one of the 

 most inspiring secrets of nature, but also owing to its manifestly 

 great practical importance. Despite vast numbers of experiments 

 and speculations very little advance was made towards discovering 

 its mechanism, or the laws of its action, till the middle of the last 

 century. Even then, when a most important clue was discovered, 

 it lay long unrecognised ; so that this clue has only been generally 

 utilised as an instrument of research since the beginning of the 

 present century. 



The clue was furnished by Mendel in 1866. He carried out his 

 pioneer experiments with Seed-Plants, and in point of fact with the 

 edible pea. The facts, that the stamens containing the pollen and 

 the carpels containing the ovules occur in the same flower, that 

 self-fertilisation normally takes place if the flowers are not interfered 

 with, and also that the stamens are easily removed without injury 

 to the carpels, were great advantages in this work. It was thus 

 possible at will to procure offspring from one parent alone, or, 

 with equal certainty, from any two given parents. 



Mendel experimented with plants which, while generally re- 

 sembling each other, differed from one another in possessing either 

 one or the other of a pair of differentiating characters (allelomorphs). 

 The object of his experiment was to observe how these characters 

 combine in the hybrid formed, and to deduce the laws according 

 to which they appear in the successive generations. For example, 

 some peas are round in contour, while others have somewhat 

 flattened faces upon them, and are angular or quadrate in form. 

 These properties are characteristic of certain varieties, and it is 

 easy to obtain strains which reproduce these differences regularly 

 in succeeding generations. In this case rotundity and angularity 

 form a pair of differentiating (mutually exclusive) characters. 

 Characters which may be thus paired in contrasted individuals are 

 called allelomorphs. It should be noted that, while these two 

 allelomorphs (roundness and angularity) are assigned to the seed, 



1 An important exception to this general rule may possibly be brought to 

 light by the line of work recently openfed up. 



