Jeffrey: Evolution by Hybridization 



27 



which are likely to have any permanent 

 value. A significant fault in our author 

 is a depreciation of morphology and 

 phylogeny, although he admits his for- 

 mer close relations with these aspects of 

 biological science. When a man past 

 middle age abandons the beliefs of his 

 most vigorous years, the critical reader 

 is not likely to have much more confi- 

 dence in his later than his earlier faith. 

 As a matter of fact, however, our 

 author's biological faith can never have 

 been very well founded, for he makes the 

 sweeping statement that all groups at 

 their period of greatest luxuriance are in 

 a condition of hybridism and in their 

 later and degenerate stage represent a 

 few purified and homozygotic species, 

 which show a repugnance to crossing. 

 He cites the cryptogamic groups in 

 proof of his assertion. The living 

 Equiseta or horsetails were represented 

 in the past by numerous and arboreal 

 Calamites, which are assumed by the 

 author, quite without proof, to have been 

 heterozygotic. The only possible evi- 

 dence in this connection is morphological 

 since the genetical analysis to which our 

 author strongly inclines is quite un- 

 available in the case of extinct plants. 

 The spores of all known Calamites are 

 uniformly well developed and do not 

 show the imperfections characteristic of 

 hybrids. There is good reason, then, for 

 regarding the ancient and more luxuriant 

 representatives of the horsetail stock as 

 quite homozygous. In the case of the 

 living survivors, however, the case is 

 quite otherwise. Here there are very 

 numerous varieties as well as recognized 

 species. One of the systematically ad- 

 mitted species, Equisetum 1 Morale Kuel, 

 on the basis of its anatomy and highly 

 abortive spores is a cross between E. 

 arvense L. and E. fluviatile L. Miss 

 Holden has shown that E. vanegatum 

 var, Jesupi is highly sterile and on 

 anatomical gounds a cross between E. 

 hiemale L. and E. variegatum Schleich. 

 Investigations proceeding in the re- 

 viewer's laboratory tend to show that 

 a number of the so-called varieties of the 

 species of Equisetum are in reality 

 nothing but hybrid forms. Thus in the 

 horsetail stock the older forms contrary 

 to Lotsy's assumption are probably 



homozygous in spite of their numbers 

 and vigor, while the few surviving 

 species of the stock are characterized 

 by a large amount of hybrid contamina- 

 tion. A similar case could be readily 

 made out for the fern stock. 



CONDITIONS FOR HYBRIDIZATION 



As a matter of fact hybridization can- 

 not occur unless conditions are favorable. 

 The lower vascular plants which owe 

 their fertilization to the agency of male 

 elements swimming in water cannot 

 freely cross if they happen to live on 

 land. The crossing of existing species of 

 the genus Equisetum is apparently 

 favored by the appendages attached to 

 the spores, which cause them to adhere 

 in clusters while in the dry condition. 

 Later, when germination takes place, 

 cross fertilization is favored by propin- 

 quity. The Calamites and their Meso- 

 zoic successors had no such appendages 

 to their spores. Among the cryptogams 

 crossing can only occasionally occur, and 

 this situation still obtains to a large 

 extent in wind pollinated forms, such as 

 the Conifers. What Lotsy elucidates as 

 favoring his hypothesis that earlier 

 groups in their plastic and luxuriant 

 phase freely hybridize in reality bears 

 quite another interpretation in the light 

 of actual history. The older forms 

 possessed a number of clearly recognized 

 characteristics, which made them un- 

 suited to modern conditions. Just why 

 these features unfitted them for actual 

 existence we do not know, but it is 

 abundantly clear in all the great lines of 

 vascular plants that those of old time 

 had an archaic organization, which in 

 every case was correlated with relative 

 extinction under modern conditions. 

 This is a sufficient reason on inductive 

 grounds, however defective it may be 

 from the purely imaginative, for their 

 having passed from the scene. Hybrid- 

 ism seems to have had little or nothing 

 to do with the matter, to judge from the 

 actual facts. To take a parallel case, 

 ancient languages were to a large extent 

 written from right to left, as for exam- 

 ple the early Latin, the most ancient 

 Greek of the age preceding Solon, the 

 hieroglyphics of the Egyptian monu- 

 ments, etc., and even many of the older 



