122 SCIENCE PROGRESS. 



alteration (to thorns) is now absent. Just in the same way 

 the bud of an oak shoot does not form a gall, except when 

 stimulated by the Cynips. 



It is needless to multiply instances further. I would only 

 remark that there exists between metamorphosis and the 

 stunted condition of an organ this much in common, namely, 

 that in both cases an arrested development of the organ 

 occurs. Organs which are gradually disappearing may be 

 recognised by the fact that they may, actually, attain varied 

 degrees of perfection, being sometimes more sometimes 

 less highly differentiated. 



Besides the results of experiments, the transitional forms 

 which occur between one structure and another afford 

 additional proof of the existence of a real metamorphosis. This 

 line of argument was exclusively followed until attention 

 became directed to the facts of actual development, and it 

 was especially associated with the study of monstrosities. 

 Teratology cannot, however, be considered here, as it 

 would require a complete article to itself. It must thus 

 suffice to mention a few instances taken from normal 

 developments. 



The inflorescences of Nidularium splendens, a Brome- 

 liad, are surrounded by a number of beautiful red leaves 

 which exhibit differences of form amongst themselves. 

 The lowest resemble the ordinary leaves of the plant, 

 save only for the red colour which occurs in their basal 

 regions. In the successive leaves the coloured area steadily 

 increases, until the entire organ is coloured from base to 

 apex ; that is, the colour-alteration affects the leaves at 

 different times in their history ; first in the later period, and 

 gradually at ever earlier stages, and it is to this fact 

 that the existence of the various transitional forms between 

 foliage leaves and bracteoles is due. In other Bromeliads, 

 for example in Bilbergia, the transition is very abrupt. 

 Instances of a similar character are common enough amongst 

 dicotyledons, and it will be sufficient to mention here the 

 cases of Eranthemum nei"vosum and Astrantia major} 



1 Other instances are quoted in my book, Vergleich. Entwickelungs 

 gesch. d. pflanzen organe. 



