198 NATURAL SCIENCE. March, 



one. If the variation is a mere increase in the definitive size of an 

 organ which has hitherto not only increased in size in the evohition 

 of the species, but also has grown during development of the individual, 

 then, of course, a variation affecting the rudiment of this organ at all 

 stages of development in which it is present, and causing it to grow 

 more rapidly, will render the late stages of development (as far as 

 that organ is concerned) more like the adult stage of the ancestor. 

 This, however, is a mere result of the mode of growth of the 

 particular organ ; its smaller size in late embryonic stages than in the 

 adult is not a result, still less is it a "record" of the course of 

 evolution. 



The antlers of stags will serve as an example. Each stag 

 develops a new pair of antlers in each successive year, and each pair 

 of antlers is larger than the pair produced in the previous year. 

 This yearly increase in the size of the antlers has been put forward 

 as an example of an ontogenetic record of past evolution. I, however, 

 deny that it is such a record. The series of ancestors may have 

 possessed larger antlers in each generation than in the generation 

 before it. It is not an occasional accidental parallelism between the 

 ontogeny and the phylogeny which I deny, but the causal relation 

 between the two. Had the ancestors had larger antlers than the 

 existing ones, there is no justification for the assumption that existing 

 stags would acquire antlers of which each pair, in later years, would 

 be smaller than those of the previous year. The yearly increase in 

 the size of the antlers is itself a character determined by Natural 

 Selection. Phylogeny appears to have run parallel with the pre-existing 

 Ontogeny. There are many breeds of hornless sheep, but they do not 

 bear large horns in early years and then shed them. If a rudiment 

 ever appears in the embryo of such sheep, its growth is very early 

 arrested. 



So it is in all the alleged cases of recapitulation. The gill-arches 

 and clefts, and the blood-vessels of an embryo bird or mammal, 

 present that striking resemblance to the corresponding parts of the 

 embryo of a fish which is expressed in Von Baer's law. They differ, 

 perhaps, only very slightly indeed. They are the modified repre- 

 sentatives of the embryonic structures of a common ancestor. 

 Whether they were, in that ancestor, the rudiments of gill-arches and 

 clefts, &c., like those of an adult fish, or not, cannot be decided by 

 embryological study. All we learn is that what now serve in their 

 modified forms as the rudiments of the gill-arches of a fish, and of 

 certain parts about the throat of a bird or a mammal, are so similar in 

 early stages of development as to show that these parts are homo- 

 logous. The greater resemblance of them to the adult structures of 

 a fish than to those of a mammal may justify the belief that they 

 served in a common ancestor as the rudiments of adult structures 

 more like the adult structures of a fish than of a mammal, and that 

 is all : they do not prove even that. 



