206 TOWARDS THE HUMAN FORM 



fundamentally the same from the very beginning ; it has been 

 modified only in the details of its canalization. 



At this point we are confronted with problematical 

 organisms, Oldhamia, not found later than the Cambrian ; the 

 Graptolites, innumerable in the Silurian Period, but extinguished 

 in the Devonian in the ramified forms called Dictyonema ; 

 Pleurodictyum, confined to the Devonian, and many others 

 that have been attributed somewhat hazardously to the 

 Hydromedusae, to Corals, or to Bryozoa. 



The fossil Oldhamia are fine ramified imprints radiating 

 from a centre {Oldhamia radiata), or diverging from the top 

 of a broken line {Oldhamia antiqua). They have been classed 

 both as Algae and as Hydroids, and it has even been thought 

 that they simply represent the trail of Worms. The fossil known 

 as Oldhamia radiata, according to this notion, came from a 

 tubicolous Worm living in the mud, which must have placed 

 the anterior part of its body in turn all round its hole. And 

 Oldhamia antiqua marked the track of the Worm which, as 

 it crawled over the mud, must have frequently changed its 

 direction, hesitating at each change and inclining the anterior 

 part of its body at different angles before deciding on its new 

 path. These changes of route would occur at such regular 

 intervals on this hypothesis as to be quite astonishing. We 

 know, too, the tracks of Annelids, in the Cambrian, at first 

 taken for Algae, and called Eophyton ; they were broad 

 and strictly rectilinear. Apart from their radiated form, not 

 a single character justifies us in interpreting Oldhamia as 

 Hydroids, and it is difficult to see in them merely ripples 

 on the surface of the Cambrian mud — another explanation 

 proposed. In short, there is no really plausible hypothesis 

 which we can adopt. 



The Graptolites lasted much longer ; they lived through 

 two geologic periods and in such great numbers that they have 

 ornamented the entire surface of certain slates. They consist 

 of minute chambers with narrow openings* arranged in close 

 formation in a single plane extending the whole length of 

 a hollow stem. These chambers may appear on one side of 

 the stem only (Monograpfiis), or on both (Diplograptus, 

 Pkyllograptus, Climacogr aphis) ; the stem may be rectilinear 

 as in the preceding genera, curved in the forms of hooks 

 {Rastrites), double and branched in the form of a printer's 



