EVOLUTION 925 



dermic notochord, of which there are only residual traces in the 

 adult. But the point is that the notochord is always quite clear in 

 the embryo, and although it may be called vestigial in the adult, 

 who dare say that it is useless in the embryo? It may serve as a 

 useful temporary scaffolding around which the backbone is built 

 up ; it may be that a backbone could not arise in individual develop- 

 ment without an antecedent, though evanescent, notochord. 



From this set of cases must be excluded those purely embryonic 

 or larval organs which give no evidence of having been much reduced. 

 Thus the "shell-breaker" or "egg-tooth" of most birds is a minute 

 transient organ which helps in breaking through the shell and then 

 drops off soon after hatching; but there is no evidence that it ever 

 was larger. But as noted above, the embryonic notochord of higher 

 Vertebrates is relatively much reduced. The cement-organ of a 

 tadpole is a very small structure which is not carried on beyond the 

 tadpole stage, but we have no warrant for saying that it ever was 

 more than larval. 



PERSISTENCE OF USELESS STRUCTURES IN EARLY 

 DEVELOPMENT. — ^Another useful distinction is between true 

 vestigial structures persisting in adult life and purely embryonic 

 useless structures which find expression in the embryonic stage's 

 only. They persist apparently because of "hereditary momentum". 

 Their hereditary "factors" are present as part of the ancestral 

 inheritance, and they give rise to parts which are of no known 

 use, and yet of no harm. They are all instances of an apparently 

 useless recapitulation of the past. Thus the embryo scorpion 

 (Euscorpius) has no fewer than seven pairs of abdominal limbs, 

 indicated by minute projecting plates. The second pair persist into 

 adult life and form the genital operculum ; the third pair become the 

 well-innervated "combs" or pectines; while all the others disappear 

 without leaving a trace. They are not known to be of any functional 

 significance; they do neither harm nor good; they are embryonic 

 relics of ancestors which had many well-developed and useful 

 abdominal appendages. Similarly, an embryo spider may show six 

 pairs of abdominal appendages, of which the last two develop into 

 the spinnerets, while all the others disappear. What can be said 

 except that a persistence of ancestral "factors" or "genes" leads to 

 an early developmental expression of six paired appendages, two 

 of which persist into adult life in very useful guise, while four 

 disappear like melting snow. 



TRANSFORMATIONS OF STRUCTURES.— This is a mode of 

 organic evolution of fundamental importance, and will be treated 

 separately, as it deserves. The reason for including a reference to it 

 in this sub-section is simply that many of the transformed organs 



