ZOOLOGY AND BOTANY. MIOBOSCOPY. ETC. 403 



without oil-globules. Most eggs are laid singly, when they have a 

 better chance of escaping their enemies than those that are laid in 

 masses surrounded by a jelly. The eggs are at first transparent, but 

 as development proceeds chromatophores appear, which then become 

 pigmented. The eggs, except those in masses, float at the surface with 

 the embryo downwards. The first fins to appear are the pectorals : 

 after the closure of the blastopore and the disappearance of Kupffer's 

 vesicle, the tail and caudal fin appear. Immediately on leaving the 

 egg the embryo is mainly dependent for locomotion on the dorsal 

 and ventral fins (leptocardiac fin) ; as growth proceeds, the pectoral 

 fins become more useful. The comparatively large size of the noto- 

 chord is a marked feature. A striking regularity is observed in the 

 appearance of the same stages of development of identical species, as 

 well as in the spawning and rate of development. 



A Suggestion from Modern Embryology.* — Mr. H. W. Conn 

 thinks that recent observations and theories may explain the difficulty 

 in the doctrine of descent, which is caused by the appearance of a 

 highly developed fauna in the Silurian epoch ; about five-sixths of 

 the orders and sub-orders now existing being represented in it. 

 Modern embryology teaches us that the various sub-kingdoms are 

 all direct modifications of the most primitive multicellular animal ; 

 the recent theories of Sedgwick " would make the history of all 

 animals much shorter by showing that all the sub-kingdoms may be 

 regarded as resulting directly from modifications of the gastrula by 

 slight changes in its shape." As slight variations at the bottom of a 

 diverging series produce much greater effects than variations higher 

 up, we may shorten the time necessary to be assumed prior to the 

 Silurian, and explain the presence of such a large number of our 

 present existing types. It must have taken a long time to develope 

 the Protozoan into the gastrula, but as soon as the latter was developed 

 various great types arose, not serially but simultaneously. Slight 

 variations in simple types would cause the descendants to separate 

 still further ; later on there would be an increase in the abundance 

 and diversity of small branches. Mr. Conn thinks that Silurian 

 vertebrates were more abundant than our present knowledge would 

 lead us to suppose. 



Evolution without Natural Selection.! — Mr. C. Dixon, basing 

 himself on a number of ornithological data, urges that isolation, 

 climatic influences, use and disuse of organs, sexual selection, and 

 interbreeding, are the determining factors in natural selection ; but 

 he forgets that the first four of these are, in the Darwinian sense, 

 factors in " natural selection." Dealing with interbreeding, he distin- 

 guishes (1) interbreeding among the individuals of a species ; (2) that 

 between sub-species, local races,and representative forms ; and (3) inter- 

 breeding " which, by absorbing a closely allied form, gradually 

 works the extinction of a species." The book appears to be most 



* Science, vi. (1885) pp. 481-2. 



t Dixon, C, • Evolution without Natural Selection ; or, the Segregation of 

 Speciea without the aid of the Darwinian Hypothesis.' 8vo, London, 1885. 



