i \i \ Mi li KIMS DIOMED I . 



In studying the question of the homolog} of the hard parte of Echino- 

 derms, we should lake them as we find them, and not attempl firs! t < • 

 build up a primordial type about which we know nothing, and which, 

 according to our theory, musl have had. or nol have had, certain hard 



part-;, ami then build up homologies i" be seriously taken with that type 

 as a starting point. While dealing with parts which have, from all we 

 now know, so slighi a connection with the internal organs, it seems natural 

 to give to the condition of the hard parts, as we find them in the fossils 

 and also in the living types and their embryos of to-day, the principal 

 weigh! in determining their affinity. 



A- a matter of course, the homologies established in the pasi have given 

 way to homologies which, according to our standpoint, represent the exist- 

 ing State of knowledge ; and we must he prepared to see our pet homolo- 

 gies demolished with more accurate information. We need hardly discuss 

 the opinion of Semon, who argues away the value of all homologies, on the 

 ground that they naturally differ in accordance with our starting point. 



It seems worse than useless to show up antiquated homologies, and print 

 such glittering generalities as "Wie es rait diesen Thatsachen aussieht 

 habe ich zu zeigen versucht, und kann mir wohl ersparen, noch andere, 

 ganz in der Luft schwebende Homologien namhaft zu machen und zuriick- 

 zuweisen" ! ! Semon closes by stating that •• Die wahrscheinlichste Anschau- 

 ung ist also die, dass die Stammform noch kein fest geordnetes Skelett 

 besessen hat," that it may have possessed a circular support for the tenta- 

 cles, and scattered plates, spicula*. or wheels in the skin, and that with 

 progressive radial development and thickening of the skin there was de- 

 veloped independently for each group of Echinoderms a type of skeleton. 

 Semon is thus maintaining far more indefinite and unsubstantiated views. 

 based upon conditions existing only in his speculations, than those reached 

 by writers homologizing the hard parts of Echinoderms. structures which 

 they find existing either in the fossil or living representatives. He is only 

 on somewhat safer ground when he comes to express the view, long enter- 

 tained by others, that such an irregular arrangement was to he found 

 among Cystideans. and strengthens his position by repeating the well known 

 fact, "dass keine Thatsache dafiir spricht dass eine Klasse durch die andere 

 Klasse hindurchgegangen ist." and that all this shows most emphatically 

 that the different classes of Echinoderms have diverged in very early geo- 

 logical times. 



12 



