t8 9 5- PALEONTOLOGY AND BIOGENETIC LAW. 309 



The lowest portion of the family tree would have to present armless 

 crowns, composed only of five basal and five oral plates, set on a 

 stalk ; then would follow genera with five large basals, five tiny radials, 

 and five stout massive orals ; then forms with five arms, at first short 

 and later on simply branched ; and so on. But I will not further 

 elaborate the picture. All know that it does not in the remotest 

 manner agree with the facts of palaeontology. What zoologist would 

 conclude from the developmental history of the recent sea-urchins, 

 that the regular forms preceded the irregular, or again that the 

 former had fossil ancestors of the type of the Perischoechinidae and 

 Bothriocidaridae ? In the ontogeny of the coelenterates there is no 

 certain indication of the former existence of Cyathophyllidae or 

 Cystiphyllidae. No observations of embryology would warrant our 

 imagining the former existence of graptolites or stromatopores. No 

 stage in the development of any living brachiopod informs us that 

 numerous spire-bearing genera lived in Palaeozoic and Mesozoic times. 

 These few instance might easily be multiplied ; but they may suffice 

 to show how trivial are the discoveries concerning existence in earlier 

 periods of earth-history that can follow from ontogenetic researches 

 alone. 



A further, indeed the practical, reason why ontogeny bears so 

 slight a relation to geology and palaeontology, lies in the fact that the 

 earlier stages of development, with which modern embryology almost 

 exclusively occupies itself, are not capable of preservation in the 

 rocks, and that we can, therefore, never expect to find their fossil 

 archetypes. The changes that occur' between the embryonic and the 

 completely adult stages have, at least among invertebrates, not yet 

 received the attention they deserve, and it is these very changes that 

 are of special interest to the palaeontologist. 



In spite of these hindrances, fossil embryonic types are not 

 entirely wanting, even among invertebrates. The palaeozoic Belinu- 

 ridae are bewilderingly like the larvae of the living Limulus ; the 

 Pentacrinoid-larva of Antcdon is nearer many fossil crinoids than is 

 the full-grown animal ; certain fossil sea-urchins permanently retain 

 such features as linear ambulacra and a pentagonal peristome, which 

 characterise the young of their living allies ; among Pelecypoda, the 

 stages of early youth of oysters and Pectinidae may be compared 

 with palaeozoic Aviculidae. Among brachiopods, according to 

 Beecher, the stages which living Terebratulidae pass through in the 

 development of their arm-skeleton correspond with a number of 

 fossil genera. Among completely distinct groups also, ontogenetic 

 characters have been successfully traced. The beautiful researches 

 of Hyatt, Wiirtenberger, and Branco have shown that all ammonites 

 and ceratites pass through a goniatite-stage, and that the inner 

 whorls of an ammonite constantly resemble, in form, ornament, and 

 suture-line, the adult condition of some previously existing genus or 

 other. 



