THE ARGUMENT FROM EMBRYOLOGY 93 



shell bears a row of tubercles near the outer side of 

 the spiral, and later still a second inner row of 

 tubercles as well, while the ribs gradually become 

 less conspicuous, and ultimately disappear. In 

 more recent forms the outer row of tubercles dis- 

 appears, then the inner row, the shell becoming 

 smooth, swollen, and almost spherical. On takino- 

 one of these smooth spherical shells, such as Aspi- 

 doceras cyclotiun, and breaking away the outer turns 

 of the spiral so as to expose the more central 

 and older turns, first an inner and then an outer 

 row of tubercles appear, which nearer the centre 

 disappear, and in the oldest part of the shell are 

 replaced by the ribs, characteristic of the earlier, and 

 presumably ancestral forms. 



Another illustration of the parallelism between 

 the pala^ontological and the developmental series is 

 afforded by the antlers of deer, which are shed 

 annually, and grow again of increased size and com- 

 plexity in each succeeding year. In the case of the 

 red-deer (Cerwis elaphus), the antlers are shed in the 

 spring, usually between the months of February and 

 April ; during the summer the new antlers sprout 

 out, and growing rapidly, attain their full size at the 

 pairing season in August or September ; they per- 

 sist through the winter, and are shed in the following 

 spring. The antlers of the first year are small and 

 unbranched ; those of the second year are larger and 

 branched ; in the antlers of the third year three 

 tynes or points are present ; in the fourth year four 

 points, and so on until the full size of the antler and 

 the full number of points are attained. 



