188 GENERAL EVOLUTION. 



peats the process again, forming another segment like the first ; 

 repetition within repetition. So with the construction of muscu- 

 lar tissue ; first, the nucleated cell repeated in a series, whose 

 adjacent walls disappear, and whose cell-contents flow together, 

 thus forming a fibrilla ; then a repetition of the same process, 

 forming a second fibrilla ; and so on to the completion of thou- 

 sands of them in fasciculi. 



Let us then trace the series of repetitions and duplicated and 

 still more complex repetitions, seen in following up animal forms 

 from their archetypes. 



In the simplest rejDetition of cell-growth in a longitudinal 

 direction we have Vibrio ; in the centrifugal, Actinophrys. The 

 former may be represented by a line of simple dots, thus : Fig. 1. 



A.^... _.. ^ 



S.y" *V ' 





J Ci ao to aQOoa ^. ooQOococ3cCf cO ^3 O 

 f" o cjqOoc5 ^rs O oodODCD£i:n c:i> o ^^O, 



\ 



In a complex repetition we rarely have the same degree of com- 

 plication in each repeated part. We have it centrifugally almost 

 perfect in a Coelenterate {Actinia), and linearly in some of the 

 lower vermes. An archetype of the latter kind might be repre- 

 sented thus : Fig. 2. In a more complex form, as of the pro- 

 glottides of TcBnia, thus : Fig. 3. The same might represent an 

 archetypal vertebrate. 



If now we attempt to express the complication of an organ by 

 modified repetition of once identical parts, the history of extremi- 

 ties will serve us. Thus the limb of Lepidosiren, which is com- 

 posed of identical segments, may be thus represented : Fig. 2. 

 Each longitudinal segment of the limb of Ichthyosaurus may be 

 similarly represented with a modification, in size only, of the 

 proximal or humerus ; thus : Fig. 4. But in Plesiosatirus an 

 important series of changes of shape (but not in complexity) ap- 

 pears, which may be represented thus : Fig. 5 ; the first being 



