II FUNDAMENTAL LAW OF BIOGENESIS 119 



palingenetio, when other organisms in adult condition possess essentially the 

 organisation of that stage of development, stands or falls with the assumption that 

 in the course of the earth's history some lower animal forms have remained very little 

 altered by the side of others which have developed into higher organisms, and that 

 it is not the case that nearly all lower animals living to-day are the degenerate 

 descendants of ancestors once highly developed. Palseontology teaches us, in fact, 

 that during those epochs of the earth's history which are accessible to investigation, 

 and among those organisms which we know in a fossilised condition, many foi-ms and 

 groups of forms have remained unaltered for immensely long periods, whfle other 

 forms on the contrary have progressed. On the other hand, we cannot doubt that 

 retrogressions in the development of the organic world are by no means rare 

 phenomena. The acquisition of a stationary manner of life, for example, and still 

 more of parasitism, necessitate such retrogressions. In many cases, in consequence of 

 new conditions, sexual maturity may be shifted to an earlier stage of development, 

 better suited for competition, and the "adult" form may gradually cease to be 

 developed. The Axolotl, for instance, generally becomes sexually mature at a stage 

 with external gills — a so-called larval stage — and reproduces itself at that stage. It 

 only rarely develops into the "adult" animal. If, in consequence of certain condi- 

 tions, the development into an adult animal were altogether and always to cease to 

 t^ke place, we should have before iis a case in which an animal, according to the 

 common conception more lowly organised, descended from one more highly 

 developed. If we wish, therefore, to compare the developmental stages of an animal 

 with the final stages of other animal forms, in order to demonstrate their palingenetio 

 meaning, we must always be' able to bring forward good reasons for believing that 

 these animal forms are not simplified or degenerated. 



If the distinction between palingenesis and ctenogenesis is difiicult, even when we 

 can illustrate it by adult animals, the difficulty iucreases when the comparison 

 remains purely ontogenetic ; i. e. when we can only compare developmental stages of 

 one animal with developmental stages of other animals. It is now accepted, that 

 when two animal groups have similar larval forms, these groups are r?,cially related. 

 The larva of Bakmoglossus agi'ees in many important points with the larva of the 

 EcMnoderm ; Balanoglossus is therefore considered to be related to the Hchinoderms. 

 It is possible that this view is correct ; but we cannot say that this relation is prob- 

 able ; for we can bring forward no reason to show that either of these larval forms 

 has any palingenetic significance. 



In dealing with ontogenetic problems we have to take into account another series 

 of considerations, which are partly a simple result of a consistent carrying out of the 

 Darwinian pirinoiples. We give only the most important. 



1. We are more likely to be justified in considering an ontogenetic process of 

 development paUngenetio, when it exhibits from first to last an unbroken series of 

 self-supporting stages of development, so that we can imagine the larva at each stage 

 to be an adult sexually mature animal. But we certainly do not find such a method 

 of development entirely realised anywhere in the animal world. 



2. It is an advantage for an animal whose organisation is adapted to its condi- 

 tions, i.e. which maintains with success the struggle for existence, to reach its adult 

 form, not only as directly and as quickly as possible, but economically, i.e. without 

 the development of parts which have become useless. 



3. Such a direct and abbreviated development can take place only when the 

 developing animal is from first to last provided with nourishment by means of which 

 it can develop. This occurs either by the egg receiving from the mother body 

 nutritive yolk to help in its development, or by its being nourished, as in viviparous 

 animals, direct from the mother body. Everything that is connected with such 



