ADDRESS. 



thesis tliat it was the embryonic repetition of an ancestral type, from 

 which all the higher forms are descended. The cavity of the cup is sup- 

 posed to be the stomach of this simple organism, and the opening of the 

 cup the mouth. The inner layer of the wall of the cup constitutes the 

 digestive membrane, and the outer the skin. To this form Hseckel gave 

 the name Gastrcea. It is, perhaps, doubtful whether the theory of 

 Lankester and Ha3ckel can be accepted in precisely the form they pro- 

 pounded it ; but it has had an important influence on the progress of 

 embryology. I cannot quit the science of embryology without allud- 

 ing to the very admirable work on ' Comparative Embryology ' by our 

 new general secretary, Mr. Balfour, and also the ' Elements of Em- 

 bryology ' which he had previously published in conjunction with Dr. M. 



Foster. 



In 1842, Steenstrup published his celebrated work on the 'Alternation 

 of Generations,' in which he showed that many species are represented by 

 two perfectly distinct types or broods, differing in form, structure, and 

 habits ; that in one of them males are entirely wanting, and that the re- 

 production is effected by fission, or by buds, which, however, are in some 

 cases structurally indistinguishable from eggs. Steenstrup's illustrations 

 were mainly taken from marine or parasitic species, of very great mterest, 

 but not generally familiar, excepting to naturalists. It has since been 

 shown that the common Cynips or Gallfly is also a case in point. It had 

 long been known that in some genera belonging to this group, males are 

 entirely wanting, and it has now been shown by Bassett, and more 

 thoroughly by Adler, that some of these species are double-brooded ; the 

 two broods having been considered as distinct genera. 



Thus an insect known as Neuroterus lenticularis, of which females 

 only occur, produces the familiar oak-spangles so common on the under 

 sides of oak-leaves, from which emerge, not Neuroterus lenticularis, but 

 an insect hitherto considered as a distinct species, belonging even to 

 a different genus, Spathegaster baccarum. In Spathegaster both sexes 

 occur ; they produce the currant-like galls found on oaks, and from these 

 galls Neuroterus is again developed. So also the King Charles oak- 

 apples produce a species known as Teras terminalis, which descends 

 to the ground, and makes small galls on the roots of the oak. From these 

 emerge an insect known as Biorhiza aptera, which again gives rise to the 

 common oak-apple. 



Many butterflies, again, are dimorphic, existing under two, or even 

 three, distinct forms— one that of the winter, the other of the summer 

 brood or broods. Weismann has adduced strong reasons for thmkmg 

 that during the glacial period these species were one-brooded only, and 

 existed in the present winter form ; that, as the climate improved, the 

 period of warmth became suflacient to allow the development of a second 

 brood, and led to the gradual rise of the summer form. 



He and Edwards have shown that, while, by the application of cold, 

 pupa3, which would naturally have produced the summer form, can be 



