172 CHAPTERS ON EVOLUTION. 



place upon development as a clue to the evolution of living beings. 

 Succinctly expressed, it is thus held by evolutionists in general, that 

 the development of the individual is a recapitulation in brief of the 

 development of its species or race. The history of the production of 

 the individual is viewed as " the abstract and brief chronicle " of the 

 changes and developments through which its race has passed in prior 

 ages of this world's existence. It is true that such a history often 

 appears meagre and imperfect. Some of its phases become altered 

 in the lapse of time by the influence of surroundings acting favour- 

 ably or the reverse upon successive generations. As the lines of 

 human progress are not always easy to trace, so those of animal 

 advance and evolution frequently appear blurred and indistinct. 

 But on the whole the record is tolerably complete. The gaps in 

 animal histories do not affect the main question at issue namely, 

 that, as Darwin says, the embryo or young animal " is a picture, more 

 or less obscured, of the progenitor, either in its adult or larval state, of 

 all the members of the same great class." That such a study must teem 

 with interest, is a remark scarcely requiring mention. Nor may it be 

 regarded as other than a triumph of scientific research, when develop- 

 ment may be seen to demonstrate how individual history repeats the 

 history of the race; and how the living world of to-day once existed in 

 simpler guise, and in the dim obscurity of the past 

 Lay hidden, as the music of the moon 

 Sleeps in the plain eggs of the nightingale. 



Although the study of animal development is in many ways an 

 intricate branch of research, there exists no difficulty in compre- 

 hending the broad outlines which mark the building of the body in 

 the higher as well as the lower forms of animal life. If we watch 

 the development of some animal such as a sponge belonging to the 

 lower grades of organisation, we may be enabled to distinguish 

 certain stages which not only mark sponge-development, but also 

 that of animal life at large. The simplest form of a sponge exists as 

 a cup-shaped body attached to some fixed object. Such a form of 

 sponge (Olynthus) is depicted in Fig. 87, 7. The walls of this cup, 

 consisting of two layers, are perforated with holes or " pores " (/), 

 which open into the substance of the cup, and thence into the interior, 

 which communicates with the outer world by the wide mouth or 

 " osculum " (os). This sponge-cup consists of two layers, of which 

 the inner is provided with delicate filaments, resembling eyelashes in 

 miniature, and named " cilia." These cilia by their constant move- 

 ment cause currents of water to flow into the sponge by the outer 

 "pores" (p\ whilst by the same movement, the water is driven 

 outwards by the mouth (os) of the cup. In this way the living 

 particles of the sponge are supplied with nutriment ; and the com- 

 parison of a sponge to a kind of submarine Venice, with canals and 



