CLOSE AND CKOSS FERTILIZATION. 447 



to be so from the observation of a series of intermediate forms 

 which bridge over the differences. Only observation can inform 

 us how much difference is compatible with a common origin. 

 The general result of observation is that plants and animals 

 breed true from generation to generation within certain somewhat 

 indeterminate limits of variation ; that those individuals which 

 resemble each other within such limits interbreed freely, while 

 those with wider differences do not. Hence, on the one hand, 

 the naturalist recognizes Varieties or differences within the 

 species, and on the other, Genera and other superior associations 

 indicative of remoter relationship of the species themselves." 



" Most varieties originate in the seed, and therefore the foun- 

 dation for them, whatever it may be, is laid in sexual reproduc- 

 tion. . . . Upon the general principle that progeny inherits or 

 tends to inherit the whole character of the parent, all varieties 

 must have a tendency to be reproduced by seed. But the in- 

 heritance of the new features of the immediate parent will com- 

 monly be overborne by atavism ; that is, the tendency to inherit 

 from grandparents, great-grandparents, etc. Atavism, acting 

 through a long line of ancestry, is generally more powerful than 

 the heredit}' of a single generation. But when the offspring does 

 inherit the peculiarities of the immediate parent, or a part of 

 them, its offspring has a redoubled tendency to do the same, and 

 the next generation still more ; for the tendencies to be like par- 

 ent, grandparent, and great-grandparent now all conspire to this 

 result and overpower the influence of a remoter ancestiy." 



1148. The reproductive elements in a complete flower may 

 combine to produce an embryo. In this case the pollen and 

 ovule have originated upon a single shoot, within very narrow 

 limits of difference as regards the time, place, and conditions of 

 their development, and the result of their union is what might 

 be expected, - - a close copy of the parent plant. The fecunda- 

 tion of a flower by its own pollen is termed close-fertilization, or 

 self-fertilization. 



1149. In cross-fertilization the pollen fertilizing the ovule of 

 a flower comes from another flower of the same species, and here 

 the reproductive elements have been developed under dissimilar 

 conditions. 



1150. In hybridization the pollen comes from a flower of a 

 different species ; and in this case the conditions, external and 



1 Volume I. pp. 318, 319. The student is urged to review carefully 

 the following sections also in that volume : 619 to 640, and 657 to 662 

 inclusive. 



