14 Arnoldia 78/3 « February 2021 
mals. Back in Paleocene, some fifty million 
years ago, mammals only started to special- 
ize in the new ecological niches that became 
available after the extinction of the dinosaurs. 
Many other winged walnut species emerged. 
Some went extinct, but the descendants of oth- 
ers are now populating the tropics of the New 
and Old World: Oreomunnea in Central and 
South America, and Engelhardia in Southeast 
Asia and northern India. It was only with the 
diversification of mammals, especially squir- 
rels, that some walnut species developed fruits 
that could be spread by animals. 
Squirrels and other rodents drove the evolu- 
tion of Juglandaceae in two different genera: 
walnuts (Juglans) and hickories (Carya), which 
evolved within separate lineages. Birds, espe- 
cially the crow family, likely played a part in 
the distribution from the beginning as well. 
Because animals never find all the nuts they 
stash in their winter storage places, they con- 
tributed to the spread of these groups, and evi- 
dently, they were quite efficient. Walnuts and 
hickories spread through North America, Asia, 
and Europe, populating much of the Northern 
Hemisphere. In the case of the walnuts, this pro- 
cess must have taken place during the span of 
about ten million years. The oldest known fos- 
sil record of the genus, a species named /Juglans 
clarnensis, was discovered in North America 
and dates back forty-four million years, while 
the oldest European specimen of J. bergomensis 
is around thirty-three million years old. 
Later, humans helped with the worldwide 
spread of two major species: the English walnut 
and the pecan (Carya illinoinensis). Whereas 
squirrels and crows spread walnuts and hicko- 
ries on three continents over several million 
years, humans extended the range of cultivation 
into all other suitable climatic regions within 
a few decades. The English walnut (a species 
of Eurasian origin) and pecan (from the south- 
eastern United States) are now cultivated well 
outside their native range, including in parts of 
South America, northern and southern Africa, 
Australia, and New Zealand. So, the tasty ker- 
nels of the walnut became the main reason for 
this widespread distribution—a process started 
by squirrels many millions of years before the 
fossil records prove the evolution of humans. 
—_ 
Today, in Central Europe, almost forty species 
and hybrids of Juglandaceae are cultivated. Dur- 
ing my research, I traveled to many parks and 
arboreta, looking for insight into the diversity 
of this family. I was driven not only by my 
scientific interest in Juglandaceae but also by 
my enthusiasm for the aesthetics of their hab- 
its, leaves, and fruits. The readers of my book 
should be able to make their own journey of 
discovery through the walnut family, on the 
tracks I have uncovered with my research. 
Often, after days of traveling, I would find out 
that a tree I wanted to visit had been cut down 
or that a rare species was simply confused with 
an ordinary, oft-planted one. I created a collec- 
tion of seeds of all the cultivated species and a 
leaf herbarium. The collection soon included 
hundreds of fruits and nuts from different loca- 
tions in Europe, which made it possible to dis- 
tinguish between the species and hybrids. Later, 
the collection became the basis for the illustra- 
tions of all species in the individual portraits 
of the book. 
These trips through Europe searching for 
the different species of the walnut family also 
brought to light the stories of other humans— 
botanists and horticulturists—who moved the 
walnut family all over the world. While I could 
find many species within a day or two of search- 
ing, many researchers spent years traveling 
through the natural habitats in North America 
and Asia a few centuries ago. In the time of 
Carl Linnaeus, only three walnut species were 
known to European researchers. Besides the 
English walnut, Linnaeus included the North 
American butternut and the eastern black 
walnut in his Species Plantarum, published 
in 1753. The hickories—especially the Asian 
species—were documented much later. 
The genus name Carya was proposed by the 
English botanist and plant collector Thomas 
Nuttall, who used the name, in 1818, in his work 
The Genera of North American Plants. He had 
borrowed this name from ancient Greek, where 
karya was a word for walnut. The valid botani- 
cal name for a genus or species should always be 
the one from the first official description, and 
in this case, Nuttall’s proposal wrongly became 
the namesake of the genus. Ten years earlier, 
