Hyatt.] 96 [March 6, 



therefore, merely suggest, because the time to follow this idea 

 out can never be ours, that research upon the later larval stages 

 and their corellations, would probably show the thickening of 

 the mesoderm and all its corellative characters to be really hom- 

 oplastic and not homogenous, 1 arising from adaptation and not 

 genetic except in small group. 



l These terms first used by Lankester (Journ. Micr. Sci., vol. xvn, 1877, p. 436) 

 express phenomena with which naturalists have long been familiar better than 

 any heretofore used, and we have accordingly adopted them in place of their 

 synonyms. 



This larva of Sycandra is an example of the most frequent and simplest illustrations 

 of concentration in development, and shows how this law differs from the ordinary 

 statement of the law of heredity. For example, Darwin writes: animals teed to 

 inherit the characteristics of their ancestors " at the same or earlier stages." The law 

 of concentration is, however, that characteristics, as a rule, arise during the adult or 

 later stages of growth and tend to be inherited at earlier ages in succeeding generations 

 of individuals, and in descendent groups of all grades. This is the unqualified general 

 statement, but the law has a medium expression in normal forms, and quite another 

 in aberrant or specialized forms. In these extreme examples the crowding of charac- 

 teristics into the younger stages causes the loss of the useless ancestral characteristics 

 in the embryo and larva. These being no longer repeated, definite evidence of the 

 genetic origin of such forms may be more or less obscured, or even entirely lost. 



Von Lendenfeld has observed the law and remarks in his Mon. Austr. Sponges 

 (Proc. Linn. Soc, N. S. Wales, vol. ix, pt. 2, p. 332), as follows: "This later kind 

 of heredity has the inclination to let peculiarities appear earlier and earlier from gene- 

 ration to generation, if these are particularly advantageous to the organism. The 

 sooner the progeny attains possession of pecularities which have shown themselves as 

 useful for ansestors the better for them." "This is the main cause of the series of 

 appearances called, "shortened heredity." The bare statement of the law of concen- 

 tration differs very slightly as given in Haeckel's Geneielle Morphologie, Cope's Ori- 

 gin of Genera, and the author's essay (Memoirs Bost. Soc. Nat. Hist., vol. i) and in 

 Von Lendenfeld's work, but all of these authors, except the writer, are united in the 

 opinion, that abbreviated development is a result of the action of Natural Selection. 

 According to our views of Natural Selection, it is not possible for it to act in 

 this way. If characteristics be divided into two classes, similarities, and differences, 

 Natural Selection is applicable only to the transmission of the clais of differences, 

 and is a law applicable to the temporary preservation of the differences between 

 organisms. It is obviously inefficient as a cause for the continued transmission of the 

 vast majority of the characteristics after they have become fixed in the structure, and 

 can be regarded as homogeneous in any genetic series. 



Heredity is the transmission of similarities alone and is perpetually and necessarily 

 opposed to the introduction of differences, adopting them only after a struggle more or 

 less prolonged. It is only in this way that we can explain such extraordinary indif- 

 ference to the transmission of such slight mutilations as the continued piercing of the 

 nose and lips among savages and the ears among civilized races, while in other cases 

 extraordinary sensitiveness is exhibited to the effects of more serious wound, or in cer- 

 tain physiological and structural results which flow from them. Our views and 



