112 LECTURES ON BIOLOGY 



of characteristics of annelids and arthropods. Like the annelids, 

 it possesses typical segmented organs, but like the arthropods, 

 in particular the insects, it has trachaea-like respiratory organs, 

 and simple yet well-defined articulated extremities, resembling 

 in their structure the parapodia of the marine chsetopods. We 

 must, therefore, assume that the ancestors of the Peripatus were 

 evolved from the annelids, and that we have to regard its family 

 as the progenitors of the myriapods and insects. 



To mention a palseontological proof, Paludina, a snail with 

 an almost complete series of forms showing its phylogeny, is 

 found in the strata of the Tertiary Period on the Island of Kos, 

 and also in Slavonia, Croatia, South Hungary, and Roumania. 

 They are distinguished by a great variability. In the earliest 

 deposits we find specimens with smooth shells and simple whorls. 

 But the higher we ascend in the succession of strata, the higher 

 is the development of the snails ; their ornamentation increases 

 in richness, differentiating them widely from the ancestral forms. 

 If we knew only the first and last link of their phylogenetic 

 chain, the stem- form, Paludina neumayri, and the most recent 

 discovery, P. hoernesi, we should certainly take them to be very 

 distant species. It is only by discovering the intermediate links 

 that it was possible to prove their gradual change from one 

 species into another. 



Similar instances are offered by other snails. Thus Planorbis 

 multiformis begins, in the oldest strata, with perfectly flat shells, 

 but evolves, in subsequent strata, more and more drawn-out 

 tower-like structures. 



The horse, rhinoceros, and tapir constitute the Sub-order of 

 Perissodactyla. Whilst the tapirs possess in their anterior ex- 

 tremities four, in the posterior, three well-developed toes, the 

 rhinoceros has anteriorly and posteriorly three toes. In the 

 horse the number has become limited to one, for it walks exclu- 

 sively on the strongly developed and elongated middle-finger, 

 while all other fingers or toes of hand and foot have degener- 

 ated, only the thin so-called ' splint-bones ' having remained to 

 prove the existence of the second and fourth digit, or, if we 

 wish to compare them with our hand, the index and ring finger. 

 It is chiefly these great differences in the formation of the 

 extremities which compel us to-day to regard the horse, rhino- 

 ceros, and tapir as members of three distinct families. 



