508 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



argument that the variations of the organisms are " determinate," and 

 are governed by the morphological possibilities, is one which surely 

 holds for any method of phylogenetic procedure. Thus it needs but 

 the briefest common-sense consideration to show that any given type 

 of leaf or flower could not possibly vary toward all other types, but 

 only in the direction of certain forms not too widely different. 



A more exi:)licit statement may prevent a misconception. It has 

 been held by some writers that variations and mutations theoretically 

 follow those already made, as a projection of them, or a continuation, 

 which carries the organs concerned successively and ever nearer some 

 ideal form or type. This is determinate variation in its strictest 

 sense. On the other hand, it is argued that from any given stage in 

 its development an organ may vary or mutate in any direction, lim- 

 ited, of course, by its morphological possibilities, and such alteration 

 may lead the structures concerned in any given course from that pre- 

 viously pursued. It need only be said that to the experimenter the 

 latter view seems to be the more fully justifiable by the facts observed 

 in mutations. 



First, it has been known for over half a century that fixed forms, 

 constant in inheritance and self-maintenant, therefore constituting 

 species, have resulted from hybridization. The fertilization of the 

 egg cell of one species by the pollen of another often results in an 

 interlocked and stable combination of the characters of the parental 

 forms in such manner as to give rise to a new type unlike either of 

 the parents, variously intermediate and constant to the new type in 

 succeeding generations. More than a thousand such fixed hybrids, 

 or hybrid species, are known, some of which have been formed anew 

 experimentally and are thus beyond doubt. It is to be seen there- 

 fore that hybridization has played, and is playing, no small part in 

 the composition of the flora of the earth, and that it must be consid- 

 ered as an active, and not unimportant, factor in the evolution of 

 plants. 



The second method by which new characters and new species have 

 been seen to arise in the succession of generations in plants is that 

 of discontinuous variation, or mutation. In following out the germi- 

 nation of hundreds of seeds in pedigreed strains of plants, a few 

 individuals may be found in each generation which are notably dif- 

 ferent from the type in anatomical and physiological features. These 

 divergencies are variously heritable, constituting breaks in descent, 

 by which, in some cases, new species originate. Such seed sports or 

 seed mutants have been seen hundreds, perhaps thousands, of times 

 during the last two centuries, but it is only within the last twenty 

 years that their phylogenetic importance began to be appreciated. It 

 will be of interest to this assembly to know that American botanists 



