GENERAL INTRODUCTION. xxxi 



probably have two separate lines of descent, one including Arachnida and 

 Crustacea, with an ancestor represented possibly by a Naztplms-form ; 

 the other Insecta, and Myriapoda with Peripatus, the latter indicating 

 perhaps an ancestry related to the segmented Vermes. There remain 

 (5) the majority of Vermes, an assemblage of apparently very diverse 

 forms ; the remarks on pp. 583-4, and the enumeration of classes, pp. 

 585-6, may suggest possible connections. 



The task of unravelling the phylogeny of the subdivisions of Metazoa 

 is one beset with extreme difficulty. The records of Geology have estab- 

 lished such points as the pedigree of the Horse, the derivation of Birds 

 from extinct Reptilians ; they show the extreme antiquity of some living 

 types, e. g. Insectans, Scorpions, Elasmobranchs in Silurian strata, the 

 great prevalence in ages past of forms now extinct or almost extinct, their 

 replacement in other instances by derived types. Other phenomena speak 

 to vast changes : the present geographical distribution of many terrestrial 

 and aquatic animals ; mimicry, migration, social habits ; the degeneration 

 which has so evidently befallen certain types, due to a sedentary mode of 

 existence, to minute size, to adaptation for a parasitic life, i. e. one de- 

 pendent on the living tissues or vital processes of another animal or plant 

 for sustained nurture, whether it be external or internal, ecto- or endo- 

 parasitism ; Alternation of Generations, first discovered by Chamisso in 

 Salpa, and by Steenstrup in Hydroids, Trematodes, &c., whether in the 

 form known as metagenesis, i. e. the alternation of asexual and sexual 

 individuals, or as heterogamy, i. e. the alternation of parthenogenetic and 

 sexual races, or in one instance (Angiostomum) of an hermaphrodite and 

 self-impregnating individual with bisexual individuals ; the occurrence of 

 prolonged metamorphoses, such as are seen in many Arthropoda, and the 

 shortened metamorphoses of the early ontogeny of most animals ; the 

 degradation of an individual into an organ a rare occurrence exemplified 

 in the avicularia and vibracula of Polyzoa, or the converse phenomenon of 

 parts of an individual becoming elevated into the semblance of a number 

 of individuals, the most probable interpretation to be put on the strobila of 

 Cestoda. Nor has natural selection left untouched the record written on 

 the pages of the life-history of any animal ; it has falsified it in various 

 ways at every stage the ovum, its segmentation, the embryo. Special 

 embryonic organs may attain a great prominence ; normal embryonic 

 phases may be slurred over, or perhaps extinguished. It is often hard to 

 say what is ancestral, what acquired, to distinguish between structures 

 which may be inherited or independently evolved. 



