360 IT'. B. Scott — Variations and Mutations. 



up what few gaps still remain. So closely do the successive 

 stages follow upon one another, that it is sometimes extremely 

 difficult to arrange them all in order and to distinguish clearly 

 those members which belong in the main line of descent and 

 those which represent incipient side-branches. Some phylo- 

 genies actually suffer from an embarrassment of riches. In 

 the case of intimately connected formations, which follow 

 upon one another with little or no break, such as the White 

 River and John Day, or in the successive strata of an uninter- 

 rupted horizon, the progress of this normal is by almost 

 imperceptible gradations, though the variations may and often 

 do describe a wide circle around the normal at every stage of 

 its descent, circles which have a striking tendency to repeat 

 themselves in the successive stages. Disregarding side 

 branches, such as Anchitheriwni, Ripparion (probably) and 

 Hippidium, the equine series from the Eocene on consists of 

 eight well defined genera, and though they are well defined in 

 one sense, the gradual change from one into the other is almost 

 without a break. The same is true of other phyla and in gen- 

 eral the continuous nature of a phylum, as at present known, 

 is apt to be proportional to the relative abundance of its repre- 

 sentatives in the strata, which is as much as to say that well 

 known series are continuous, while apparently discontinuous 

 series are those which are imperfectly known. Indeed, the 

 most striking fact about any well established phylum is the 

 directness of its advance towards its final goal, however greatly 

 or in however many different directions it may be varying at 

 any or all stages of its history. This does not imply that a 

 line may not give off side branches or may not bifurcate into 

 two lines. But while we represent such ramifications by 

 branching lines in our genealogical trees, it must not be for- 

 gotten that each of the branches, considered only with refer- 

 ence to itself, forms a direct line from ancestor to descendant. 

 Continuity in the advance of a series of animals is not pecu- 

 liar to mammals ; it recurs wherever the phyla can be worked 

 out in detail. Thus Waagen says of Ammonites : " Sehr 

 hiiufig zeigt sich namlich bei den hierher gehorigen Ammo- 

 niten, dass mehrere auf einander folgende Schichten Formen 

 ein und desselben Bildungstypus beherbergen, welche einander 

 ausserst nahe stehen, die mit einander naher verwandt sind, als 

 mit alien iibrigen in den gleichen Schichten liegenden Arten, 

 bei denen endlich nur bei sehr eingehenden Studien und sehr 

 reichhaltigem Materiale endlich Unterschiede gef unden werden 

 konnen, die sich in alien Fallen alsstichhaltig erweisen. Solche 

 Bildungstypen kann man oft durch eine grosse Zahl von 

 Schichten hindurch verfolgen, aber in jeder Schicht zeigen die 

 Individuen eine von den vorbergehenden und nachfolgenden 



