VARIATION. ADAPTATION. ORIGIN OF SPECIES 383 



Winkler. 1902. Bot. Ztg. 60, II. Abt., 264. 

 Winkler. 1903. Ber. d. bot. Gesell. 2 1 , 96. 

 Zacharias. 1901. Ber. d. bot. Gesell. 19, 2)77' 



LECTURE XXX 

 VARIATION. ADAPTATION. ORIGIN OF SPECIES 



(See note beneath the title of Lecture XXIX.) 



In the last lecture we considered the subject of hybridity with the view of 

 gaining some acquaintance with the significance of fertilization. The facts 

 brought forward introduced, however, another problem of even greater interest, 

 viz. that of the transmission of characters in reproduction. It maybe shown that 

 when hybridization is effected there is a hereditary transmission of the pecu- 

 liarities of two organisms, so that the offspring differs in appearance from both 

 parents. Although the variations so induced are often by no means permanent, 

 either because segregation occurs or because many hybrids are sterile and hence 

 produce no offspring at all, there are, on the other hand, hybrids, both natural 

 and artificial, which are completely fertile, and in which the deviations from 

 the parental types are permanent, or, in other words, new plant forms may arise by 

 hybridization. Two questions now suggest themselves ; in the first place, are 

 the multitudinous forms which we meet with in nature the result of crossing 

 of a few original types ? And, again, is crossing the only cause of variation in 

 nature, or, conversely, were it not for this crossing, would the offspring produced 

 sexually or asexually always resemble their parents in all essentials ? 



If for the term ' types ' we read ' species ', we have stated the problem, 

 par excellence, which has exercised the minds of biologists since the middle 

 of the last century, viz. the problem of the origin of species. In the treat- 

 ment of this problem biologists have collected an overwhelming amount of evi- 

 dence, zoological, botanical, and palaeontological, all tending to show that the 

 species which now inhabit the surface of the earth have been derived from other 

 different types which previously existed on it. This view has been briefly 

 termed the Theory of Descent, a theory which, as is well known, was brought 

 into prominent notice by the immortal Charles Darwin, who succeeded where 

 previous supporters of the theory were less fortunate. The facts on which the 

 Theory of Descent is based are essentially morphological and systematic, but 

 the experiences of the gardener and the agriculturist have also proved of immense 

 value as corroborative evidence. Plant physiology, however, has in the past 

 been only to a limited extent concerned with this problem, and it is only of 

 recent years that it has been recognized that experimental physiology might 

 aid in the solution of such problems. It is obvious that physiology alone is 

 able to shed light on the modifications which species undergo, and to elucidate 

 the more immediate conditions of such changes — to answer the questions how ? 

 and why ? Of course experiment cannot tell us how the present vegetation came 

 into existence — that is an historical question whose solution cannot be reached 

 even in the most elementary form, since the necessary documents are available 

 only to the most meagre extent. The task of physiology is to study the 

 changes which go on in plants at present living, to measure their extent, and 

 to investigate their causes. From that study one may draw conclusions as to 

 the phenomena which took place in earlier periods of the earth's history, and 

 so it is possible for physiology and morphology to work hand in hand in this 

 field of inquiry. 



