June 20, 1902.] 



SCIENCE. 



977 



joyed wider acceptance than these: That 

 higher animals in their development from 

 the fii"st beginnings correspond, stage for 

 stage, with the adult condition of lower; 

 that the development of the individual 

 takes place according to the same laws 

 as that of the series; that the more 

 highly organized ones pass in general 

 through the adult stages of those less 

 highly organized, so that the differences 

 between the stages in individual develop- 

 ment, may be referred back to the differ- 

 ences between persisting adult forms. 



These opinions, born of the time when, 

 excepting Malpighi and Wolff, no one had 

 studied connectedly the earlier periods in 

 the history of the development of any 

 animal, could not fail to excite interest; 

 particularly since by their aid certain mal- 

 formations could readily be explained as 

 cases arrested in development. The rampant 

 speculations of the Lamarckians derived 

 support from them, but the teachings of 

 this school were as repugnant to von Baer 

 as to many other thoughtful students. 



Suppose, he says, that a fish stranded on 

 a sandy beach were seized with the desire 

 to Avalk, then, according to this school, the 

 fins, unsuited for the perambulatory move- 

 ments, would promptly shrink in breadth 

 from disuse and in turn grow in length. 

 These modified appendages, transferred to 

 children and grandchildren for several 

 thousand generations, are naturally in the 

 end transformed into feet. Naturally, too, 

 the fish in the meadow gasp for air, and 

 their struggles in the end produce lungs, 

 the only requirement being that a few 

 generations should be exposed to the slight 

 inconvenience of not breathing at all. 



The long neck of the heron is due to the 

 fact that his ancestors often stretched that 

 organ in order to catch fish. Their chil- 

 dren came into the world with elongated 

 necks and the same evil habit, and thus 

 gave to their offspring necks still longer. 



from which it follows that if our planet 

 only reaches a ripe old age, the heron's 

 neck ^vill extend beyond the bounds of cer- 

 tain knowledge. 



II. 



DOUBTS AND OBJECTIONS. 



(a) At an early time von Baer saw that 

 the relationships between different animals 

 could not be looked upon as representing 

 a steady advance, which is a necessary 

 corollary of the propositions he has set out 

 to criticise. Above all, suspicions were 

 generated from the fact that until that 

 time only the development of the higher 

 forms was known, and this incompletely. 

 What differences their embryonic history 

 exhibited must, if they were to fiad analo- 

 gies anywhere, find them among the lower 

 animals. Indeed, resemblances between the 

 embryonic condition of certain animals, 

 and the adult stages of others, seemed to 

 von Baer quite necessary and without sig- 

 nificance, since they all fall within the 

 realm of the animal kingdom, and the 

 variations of which the animal body is 

 capable are determined in each case by the 

 interrelations of the separate organs, and 

 in these interrelations, repetitions neces- 

 sarily occur. 



If birds had studied their own embryonic 

 history, and were now engaged in unravel- 

 ing the structure of adult mammals and 

 man, would not their test-books read as 

 follows : 



Those four- and two-legged animals have 

 similar embryos, for the bones of their 

 skulls are separate and they have no bills, 

 as we have after five or sis days of incuba- 

 tion. Their extremities are all pretty much 

 alike, about the same in length as our own ; 

 not a single true feather adorns their 

 bodies, but only a thin do^vn, in which 

 respect our very nestlings surpass them. 

 Their bones are not very brittle and con- 

 tain (as ours do in youth) no air; indeed, 



