GENERALISATIONS. 69 



Von Baer, one of the pioneer embryologists, acknow- 

 ledged that, with several very young embryos of higher 

 Vertebrates before him, he could not tell one from the 

 other. Progress in development, he said, was from a general 

 to a special type. In its earliest stage every organism has 

 a great number of characters in common with other 

 organisms in their earliest stages ; at each successive 

 stage the series of embryos which it resembles is nar- 

 rowed. The rabbit begins like a Protozoon as a single 

 cell; after a while it may be compared to the young 

 stage of a very simple vertebrate ; afterwards, to the young 

 stage of a reptile ; afterwards, to the young stage of almost 

 any mammal; afterwards, to the young stage of almost 

 any rodent; eventually it becomes unmistakably a young 

 rabbit. 



Herbert Spencer expressed the same idea, by saying that 

 the progress of development was from homogeneous to 

 heterogeneous, through steps in which the individual history 

 was parallel to that of the race. But Haeckel has illustrated 

 the idea more vividly, and summed it up more tersely, than 

 any other naturalist. His " fundamental biogenetic law " 

 reads : " Ontogeny, or the development of the individual, is 

 a shortened recapitulation of phytogeny, or the evolution of 

 the race." 



It is hardly necessary to say that the young mammal is 

 never like a worm, or a fish, or a reptile. It is at most like 

 the embryonic stages of these, and it may also be noticed 

 that, as our knowledge is becoming more intimate, the 

 individual peculiarities of different embryos are becoming 

 more evident. Thus Professor Sedgwick has recently said 

 that a blind man could distinguish the early stages of 

 Elasmobranch and Bird embryos. But this need not lead 

 us to deny the general resemblance. 



Moreover, the individual life history is much shortened 

 compared with that of the race. Not merely does the one 

 take place in days, while the other has progressed through 

 ages, but stages are often skipped, and short cuts are dis- 

 covered. And again, many young animals, especially those 

 " larvse " which are very unlike their parents, often exhibit 

 characters which are secondary adaptations to modes of life 

 of which their ancestors had probably no experience. In 



