HARDWICKE'S SCIENCE-GOSSIP. 



199 



The chief group of fossil-sponges is called Petro- 

 spongiadae (irerpos, stone). Of this group, Sparsi- 

 spongia is found in the Devonian, and Ventriculites in 

 the chalk formations. 



Lower Silurian sponges are Pakeospongia and 

 Acanthospongia ; Sparsispangia is found frequently in 

 the Devonian. As already observed, the chalk is 

 richest in sponges, and the most important genera 

 there occurring are Siphonia and the aforenamed 

 Vent7-icirfites. It is a curious fact that the fauna of the 

 Cretaceous epoch is reproduced, as it were, at great 

 depths in the ocean ; for at these great depths Fora- 

 minifera, Crinoidea (Echinoderms), and Spongidre 

 are associated in a manner highly characteristic of 

 the chalk formation. 



A few words must be added on the classification 

 of sponges. And it must be confessed that this part 

 of the subject is far from being satisfactorily worked 

 out. The arrangement of Dr. Bowerbank is the 

 one generally adopted. In this classification three 

 orders are formed ; the nature of the skeleton being 

 used as the classificatory medium. The first order 

 Keratosa, comprises the horny sponges, whose 

 skeletons are furnished in a greater or less degree 

 with spicula. This order is generally regarded as 

 the lowest. The second order, Calcarea, includes the 

 sponges whose skeleton is composed of lime, and the 

 third order, Silicea, comprises the siliceous or flinty 

 sponges. In these last the skeleton is made up either 

 of continuous interlacing siliceous fibres, or else of 

 siliceous spicules. The form of the spicules is subject 

 to great modifications ; though in the same part of the 

 animal they are constant for a particular species. 

 They form interesting microscopical objects, the 

 different forms being so well marked and manifold 

 in number. 



In conclusion, it may be noticed that fossil sponges 

 are more intimately allied to the forms which now 

 flourish in tropical countries than to other kinds. 

 And hence we see one proof of the fact, which is also 

 deducible from other phenomena, that at the epoch 

 when these sponges existed as living beings, a climate 

 prevailed over a great portion of the globe which 

 must have been very conformable to that now existing 

 in tropical countries. So that, from the study of the 

 life-history of such humble creatures as the sponges, 

 we are led to infer that the climate of the earth was 

 not always as we now find it, but that tropical heat 

 once called forth a luxuriant flora and fauna in 

 districts where now arid sterility and desolation pre- 

 vail, and that icy seas and inhospitable shores now 

 exist where formerly sponges flourished, and the 

 ocean teemed with animal life in a manner somewhat 

 similar to what we now see in the tropical oceans of 

 the world. Considerations such as these would cer- 

 tainly point out to us the advisability of carefully 

 attending to the anatomy and functions of the lower 

 orders of animals and plants ; for it has been in the 

 past, and will be in the future, that observations on 



these points, carried out on strictly inductive prin- 

 ciples, have led to some of the most magnificent and 

 comprehensive discoveries and generalisations by 

 which the progress of science has been advanced. 



INTERESTING PLANTS IN THE ROYAL 

 GARDENS, KEW. 



ONE of the greatest curiosities of the vegetable 

 world is now growing in these gardens, viz. 

 IVehoitschia mirabilis, and although dried specimens 

 of this remarkable plant are frequently seen, it has 

 never been the good fortune of any person to see a 

 living plant of moderate size in this country before- 

 Seeds have been received at Kew which have 

 germinated, but, from unknown causes, they have 

 invariably died shortly afterwards. The specimen 

 under consideration was imported a few months since, 

 and after being carefully attended for some time has 

 lately commenced a somewhat uncertain growth. 

 This wonderful plant was discovered by the late 

 Dr. Welwitsch on the elevated dry sandy plateau, 

 near the Benguela coast of West Tropical Africa, 

 where it forms a peculiar feature in the landscape 

 The stems rarely exceed one foot in height, but 

 frequently attain a diameter of three feet, and the two 

 cotyledons are, with the exception of the flowers, the 

 only appendages produced, and these remain attached 

 to the stems and continue to increase in length during 

 the whole life of the plant, which in its native country 

 exceeds a hundred years. On old plants the coty- 

 ledons are more than six feet long, and about two or 

 three in width, and they are torn by the wind into 

 long strips which trail on the ground, and flutter 

 about in every gust that blows. From the margin of 

 the stem above the cotyledons rises the dichotomous 

 inflorescence on the divisions of which are borne 

 terminal cones composed of imbricated scarlet bracts 

 in four rows, each bract enclosing an extremely simple 

 flower. After flowering, the cones increase to about 

 two inches in length. It can readily be imagined how 

 strange these stunted stems must appear dotted about 

 over a level country, but we can scarcely realise the 

 feelinss of Dr. Welwitsch when he first beheld this 

 abnormal member of the vegetable kingdom. The 

 specimen growing at Kew was recently exhibited at a 

 meeting of the Linnean Society, when it attracted 

 considerable attention, as it was the first living plant 

 ever exhibited before the society, and more especially 

 as the plant is not in very vigorous health, and therefore 

 the duration of its life may be rather short. From 

 observations made at Kew the growth of the cotyledon 

 appears to be entirely basal, and to proceed at the 

 rate of about 5-10 mm. per month. We might 

 mention that considerable doubt existed at one time 

 concerning the classification of the Welwitschia, but 

 Dr. Hooker some time since definitely referred it 

 to Gnetacerc, a tribe of Conifers?. 



Lewis Castle. 



