FINAL REMARKS 493 
relationship to monkeys in general. By this method very evident 
blood-relationships have been demonstrated, especially between 
members of the Mammalia. 
I therefore started upon an investigation into the possibility of 
proving relationship in this way between Limulus and Ammoceetes, 
with the kind assistance of Mr. Graham Smith. I must confess I 
was not sanguine of success, as I thought the distance between 
Limulus and Ammoccetes was too great. Dr. Lee, of New York, 
kindly provided me with most excellent serum of Limulus, and 
the first experiments showed that the anti-serum of Limulus gave a 
most powerful precipitate with its own serum. Graham Smith then 
tried this anti-serum of Limulus with the serum of Ammoccetes, and 
to his surprise, and mine, he obtained a distinct cloudiness, indicative 
of a relationship between the two animals. This, however, is not 
considered sufficient, the reverse experiment must also succeed. I 
therefore, with Graham Smith, obtained a considerable amount of 
blood from the adult lampreys at Brandon, and produced an anti- 
serum of Petromyzon, which gave some precipitate with its own 
serum, but not a very powerful one. This anti-serum tried with 
Limulus gave no result whatever, but at the same time it gave no 
result with serum from Ammoceetes, so that the experiment not only 
showed that Petromyzon was not related to Limulus, but also was 
not related to its own larval form, which is absurd. 
Considerable difficulties were encountered in preparing the 
Petromyzon anti-serum owing to the extreme toxic character of the 
lamprey’s serum to the rabbit; in this respect it resembled that of 
the eel. It is possible that the failure of the lamprey’s anti-serum 
was due to the necessity of heating the serum sufficiently to do 
away with its toxicity before injecting it into the rabbit. At this 
point the experiments have been at present left. It will require a 
long and careful investigation before it is possible to speak decisively 
one way or the other. At present the experiment is positive to a 
certain extent, and also negative; but the latter proves too much, for 
it proves that the larva is not related to the adult. 
Some day I hope this “biological test” will be of use for 
determining the relationships of the Tunicata, the Enteropneusta, 
Amphioxus, etc., as well as of Limulus and Ammoceetes. 
The origin of Vertebrates from a Paleostracan stock, as put 
forward in this book, gives no indication of the systematic position 
