MENDELISM 223 



MENDELISM AND EVOLUTION 



You may remember one curious misconception the 

 early botanists had in connection with the phenomena of 

 reproduction. Even Schleiden thought that the plant 

 embryo was carried by the pollen-tube to the ovule, 

 whose chief duty it was to form a nidus in which the 

 embryo might be properly fed and protected. The 

 male was the source of the " seed/' the female was the 

 soil in which it was sown. This idea accounts for the 

 use of the word " seed " in two senses the ancient one, 

 as signifying the germ of a new generation that always 

 came from the male (conf. " The seed of David " and 

 the Bible passim), and the modern one, indicating an 

 embryo plant with its protective coverings. It was 

 only after the invention of the microscope and the dis- 

 covery of egg cells and fertilising cells that it was 

 appreciated that the embryo was the product of fusion 

 of constituent units from two individuals, or from two 

 different parts of the same individual. Both of these 

 units were of microscopic dimensions, although the female 

 ovum was, as a rule, many times the size of the male sperm. 

 Minute as these units were, the marvellous fact remained 

 that, in some way or another, the characteristics of the 

 parents were reproduced in the offspring. That " like tends 

 to beget like " was an ancient truism, and it signified that 

 the multifarious characters of the parents, both material 

 and immaterial, were transmissible to their offspring, i.e. 

 were inherited. This inheritance of parental characters did 

 not exclude the appearance of individual variations in the 

 offspring, nor indeed the resuscitation of features of some 

 far-back ancestor, a phenomenon known as " atavism." 



Of course the only possible agents by which the 

 parental or ancestral characters could be transmitted 

 were the germ cells, and when the microscope had become 

 sufficiently perfected to reveal the presence of chromosomes 

 in the nucleus and to enable us to follow the remarkable 



