THE DEVELOPMENT OF ORGANISMS 741 



tion of living and extinct animals. Though no evolutionist, he 

 wrote : 



"It may therefore be considered as a general fact, very likely to 

 be more fully illustrated as investigators cover a wider ground, that 

 the phases of development of all living animals correspond to the 

 order of succession of their extinct representatives in past 

 geological times." 



His son, Alexander Agassiz, compared stages in the development 

 of Echinoderms with the fossil series, and said: 



"Comparing the embryonic development with the palseonto- 

 logical one, we find a remarkable similarity." 



To Haeckel, in particular, credit is due for recognising the 

 importance of the recapitulation doctrine and stating it clearly in 

 the light of evolution. He called it "the fundamental law of bio- 

 genesis" ("biogenetisches Grundgesetz"), and stated it in the 

 familiar words: "Ontogeny is a recapitulation of phylogeny." He 

 also introduced the idea of palingenetic characters, which correspond 

 to those of the ancestral stock, and kainogenetic characters, which 

 are relatively recent additions. The latter, he said, may disguise the 

 former in a perplexing way; in any case, the recapitulation is 

 general, not exact, and often shows great condensation. Fritz 

 Miiller was another who did much (e.g. in his Fiir Darwin, Leipzig, 

 1864) to illustrate and corroborate the recapitulation idea. 



This doctrine has suffered considerably at the hands of its friends, 

 who have sometimes stated it in an exaggerated and inaccurate 

 way. When Milnes Marshall said, "Every animal in its own develop- 

 ment repeats its history, climbs up its own genealogical tree", he 

 was speaking picturesquely, for the recapitulation is general, not 

 detailed; it often shows telescoping, and it is truer of stages in 

 organogenesis than of stages in the development of the embryo as a 

 whole. It is hardly necessary to say that a developing bird is never 

 like a reptile, but only like an embryo reptile. It has also to be 

 remembered that one term in the comparison, the phylogeny, is 

 very imperfectly known, so that assertions as to the exactness of 

 the recapitulation must be taken with much reserve. And, again, 

 the illustrations that have been adduced have not always been very 

 happy. The simplest animals are like single cells ; there are some balls 

 of cells, like Volvox, on the border-line between unicellulars and 

 multicellulars; and there are some very simple two-layered sacs of 

 cells, such as Protoh^^dra. But, when we see an animal of relatively 

 high degree, such as the primitive vertebrate Amphioxus, beginning 

 its life as a fertilised egg-cell, which develops into a ball of cells 

 (blastula) and a two-layered sac of cells (gastrula), we are probably 

 mistaken in regarding this as a recapitulation of very ancient phylo- 

 geny. Reproduction by means of isolated germ-cells need not have 

 any historical reference to the Protozoa; a ball of cells may be the 



