C— GEOLOGY 67 



characters, superposed upon the stable embryonic foundation. These 

 undergo, with comparative rapidity, a course of evolution the stages of 

 which are very completely recapitulated during development. The fact 

 must be emphasised that in so far as specific and complete adult recapitula- 

 tion takes place it seems, in the example before us, to be limited to the 

 neanic phase. 



In the controversy briefly referred to at the outset biologists, in 

 discussing the problems before us, have based their arguments almost 

 entirely upon embryonic, larval or foetal material. Palasontologists, on 

 the other hand, have rarely had such material at their disposal, for such 

 early developmental stages are either not capable of preservation in the 

 fossil state, or they are such minute and delicate objects as the prodisso- 

 conch of lamellibranchs, the protoconchs of gastropods and cephalopods, 

 the protaspids of trilobites, which are easily destroyed. The palasonto- 

 logist's evidence therefore is usually drawn from neanic stages which, it 

 may be noted, make up the major portion of the individual life-history 

 and are more abundantly preserved in the fossil state. Inasmuch, 

 therefore, as these two classes of workers are on the whole dealing with 

 different portions of that life-history, their observations and the con- 

 clusions they draw are not contradictory but supplementary. As far as 

 our study of Zaphrentis takes us we may say that the embryonic stages of 

 development recapitulate the changes exhibited by corresponding stages 

 of other forms belonging to the same general stock, and that the neanic 

 stages recapitulate the adult condition exhibited by the preceding members 

 of the gens to which the species belongs. Further, within the neanic 

 stages the principle of acceleration or tachygenesis is perfectly exemplified, 

 but its action, so far as the adult combination of features is concerned, 

 does not penetrate back into the transitional and embryonic stages. In 

 these latter the rate of acceleration does not remain the same for all 

 features and consequently the adult combination undergoes disruption. 



In the series Z. delanouei [s. str.)-Z. constricta the development of the 

 later members runs parallel to but overlaps beyond that of the earlier. 

 But with Z. disjuncta new tendencies enter, and though the earlier, 

 typical and later members of this species exhibit in their development 

 a like parallelism and overlapping, the direction they follow diverges from 

 that of the former members of the gens. By acceleration these new 

 tendencies ultimately cut out the older combination almost completely 

 from the developmental record of the advanced members of Z. disjuncta. 

 Here then is a very clear case of ' skipping of stages ' or lipopalingenesis 

 of the kind referred to by Trueman as a ' straightening of ontogeny ' as 

 opposed to ' mere shortening of ontogeny ' which results from tachy- 

 genesis. It should be noted that in this case the straightening is rendered 

 necessary by the fact that divergent changes had previously set in. 



Other Examples of Recapitulation. 



With this general discussion, suggested by the study of Z. delanouei 

 {s. lat.), as a basis to work upon we must now inquire whether the same 

 general phenomena are recognisable in the development of other types 



