UG 



Mr. W. T. Thiselton Dyer. 



believe we shall have no trouble in obtaining crested flowers in a 

 variety of colonrs." 



The corolla segments of Cyclamen have no mid-rib. The appearance 

 of such a structure is a reversion to the original leaf-type. The 

 development of a crest from a mid-rib carries reversion very far back 

 indeed. The branching of a leaf-structure in the plane in which it is 

 expanded is common enough ; branching in a plane at right angles 

 to this is rare. Leafy outgrowths frequently occur from the 

 raid-rib in the cabbage.* In this case the structure of the leaf 

 approximates to that of a stem, of which, indeed, the leaf may be 

 regarded as a modification. 



An interesting fact with regard to this singular variation is that 

 it has appeared more than once, and independently. It first occurred 

 in 1885, but seems afterwards to have been lost sight of.f It has 

 also occurred in a red-flowered form in France, J in which case it was 

 also perpetuated by seed. 



I have not succeeded in discovering any similar structure in any 

 primulaceous plant occurring in a wild state. Dr. Masters, how- 

 ever, informs me that it has been observed in cultivated forms of 

 Primula sinensis. The tendency thus seems to be latent in the order, 

 though why it should be so I am unable to explain. 



Some theoretical interest appears to me to attach to the rapid 

 development of so striking an ornament of a corolla segment. Such 

 appendages are frequent enough in orchids, and are regarded as 

 adaptations to cross-fertilisation by insects. Their gradual evolution 

 might be thought to require a long period of time : but in the 

 present case we have definite evidence that such a structure may be 

 developed by selection with great rapidity. 



Conclusion. 



1. The facts which I have stated appear to me to establish the 

 result that when once specific stability§ has been broken down in a 

 plant, morphological changes of great variety and magnitude can be 

 brought about in a comparatively short space of time. This appears 

 to me to have a very important bearing on the rate of evolution. 

 Mr. Darwin quotes Lord Kelvin as insisting "that the world at a 

 very early period was subjected to more rapid and violent changes in 

 its physical condition than those now occurring;" and he adds, 

 4i Such changes would have tended to induce changes at a corre- 



* Masters' ' Teratology,' p. 455. 



f 1 G-ardener's Chronicle,' 1885, p. 536. 



t ' Revue Horticole,' 1897, pp. 98 and 130. 



§ For a general discussion of the principles of variation and specific stability, 

 see ' Nature,' vol. 51, pp. 459 — 4C1. 



