154 PACKARD—CLASSIFICATION OF ARTHROPODA.  [April3, 
3. The compound eyes of the Merostomes became broken up 
into groups of single eyes. 
4. Most remarkable changes took place in the internal organs, 
resulting in the development of salivary glands, none occurring in 
Crustacea and other marine Arthropods. 
5. The acquisition of Malphigian or urinary tubes which exist 
in terrestrial Arthropods, Arachnida, insects, etc., but in no 
marine Arthropods. 
6. A gradual reduction in the number of pairs of nephridia, all 
Arachnida having but a single pair, Limulus having four pairs, and 
the Eurypterida presumably as many. 
7. After the scorpion type became fixed and the spiders arose, 
the number of pairs of book-lungs became reduced from two pairs 
in Mygale to one in other spiders, and then began an evolution of 
tracheze from dermal glands—a process seen in certain terrestrial 
planarian worms as well as land-leeches. 
8. While the arterial system of Limulus, owing to its localized 
organs of respiration, is remarkably developed, in the scorpion the 
arterial system is greatly reduced, and in the tracheate Arachnida, 
such as the spiders, there are no arteries or venous lacune. 
It is most probable that the evolution of the Palzeophonus descen- 
dants, viz., the scorpions of the Carboniferous—assuming that they 
were true scorpions—took place with comparative rapidity, z. e., by 
tachygenesis, without the extremely slow method postulated by the 
natural selectionists, the modification suggested above having con- 
temporaneously affected all the individuals, many thousands or tens 
of thousands alike. The method was not, as Darwin imagined, the 
result of a single chance variation gradually and by numberless 
intermediate forms passing into a species which gave origin to 
many others, from which were gradually evolved new subgenera, 
genera, subfamilies and so on, but the method was radically differ- 
ent. The Paleophonus, an Eurypterid, became, we take it, in a 
comparatively few generations the parent of a scorpion, the repre- 
sentative of a distinct class. The class characters, great as are the 
differences, especially in its internal organs, between an Arachnid 
and a Merostome, were assumed with comparative suddenness. 
New classes, like new species, did not arise from a single but from 
a large number of individuals. This was Lamarck’s doctrine, and 
it has been reaffirmed by De Vries. This shows that even classes 
