92 PRIMITIVE FORMS OF LIFE 



Unfortunately, it is not quite so simple as this, for 

 within those apparently innocent words of Haeckel's formula — 

 "abbreviated recapitulation" — are concealed such insidious 

 pitfalls that Haeckel himself could not avoid them, and 

 could not even succeed in showing us where to look for them. 



As we have already stated, the duration of life on earth is to 

 be reckoned in millions of years, and a similar length of time 

 must be assumed for existing species to have acquired their 

 special characters. Now the maximum period required for the 

 development of an individual animal, apart from size, does not 

 exceed two years, and certain very complex insects complete 

 their term of life, including their metamorphoses, in a few 

 weeks. In the one case as in the other, the abbreviation of 

 descent that takes place in embryogeny is simply stupendous. 

 It is, moreover, exceedingly unequal, even in closely related 

 species. There are instances in which a young animal is hatched 

 with only a small portion of its body developed : the other parts 

 are then successively formed without cessation of the normal 

 life-activity. There is consequently always a possibility that 

 the successive forms which the creature assumes will represent 

 ancestral forms, or at least forms which suggest them. To the 

 modes of development which fulfil this condition we give the 

 name of patrogony. However, even here the abbreviation is so 

 great, and the ancestral forms succeed one another so rapidly, 

 that they are telescoped into one another, so to speak, and the 

 successive shapes assumed by the embryo may be considered 

 only as analogous to ancestral forms and not their exact 

 reproduction. The general trend of evolution is here indicated, 

 not its detail. 



In some species related to those described above the young 

 animal emerges, if not in possession of its final form, at all 

 events provided with all the parts of its body. The develop- 

 ment then takes place so quickly that the parts of the 

 body which were formed successively in the preceding case, 

 here appear to be formed simultaneously ; thus the course of 

 evolutionary development can be so modified, and the forms 

 assumed by the embryo recall the ancestral forms so little that, 

 if at any given moment we were to free it from its protecting 

 envelope, the organism would be unable to lead an independent 

 existence. This is the case with all vertebrates except 

 Amphioxus. We call this greatly accelerated embryogenetic 



