Question: When did he join the Kahnweiler art gallery?
About the article "Les Demoiselles d'Avignon", section "Rivalry with Matisse".

Candidate passages:
Kahnweiler is considered one of the major dealers and spokesmen for Cubism.
He was among the first people to recognize the importance and beauty of Picasso's "Les Demoiselles D'Avignon", immediately wanting to buy it and all of Picasso's works.
Picasso wrote of Kahnweiler "What would have become of us if Kahnweiler hadn't had a business sense?"
– compelling because at the time of the origination of his most famous works Picasso was largely unknown as an artist and destitute.
Kahnweiler supported in his gallery all of his great artists of the time; who had no audience, or collectors, - initial purchases included works by Kees van Dongen, André Derain, Fernand Léger, Georges Braque, Juan Gris, Maurice de Vlaminck and several others of the same generation.
Kahnweiler wanted as he said to "defend" artists he believed in, but only those who had no dealers and whom he believed were great artists.
Rather than exhibiting the popular works of the past and present greats, Kahnweiler championed burgeoning artists such as André Derain, Alberto Giacometti, and others, who had come from all over the globe to live and work in Montparnasse and Montmartre at the time.
Thus Paul Cézanne although a great artist was considered too old to be represented, and his work was being handled by Ambroise Vollard at the time.
Along with such men as Alfred Flechtheim, Paul Cassirer, Daniel Wildenstein, Léonce Rosenberg and Paul Rosenberg, Kahnweiler was one of the influential connoisseurs of the 20th century.
As a businessman Kahnweiler innovated many new ways of working with artists; and art dealing; - now established practice with art dealers.
In 1907 when there were only half a dozen good galleries in Paris, he made contracts with artists to buy all of their work (freeing them from the worry and restraint about cost of living and creating work).
He saw them daily to discuss their work, (they were friends, he co-owned little sailing boats with his artists), he photographed each work, (he felt it imperative to have a record), he held exhibitions of their work and he promoted their work internationally.
Kahnweiler gave the first exhibition of the work of Georges Braque.
He encouraged the practice of "Beau Livres" (beautiful book), where a contemporary artist illustrates a work of a contemporary writer - he expanded his presentations by bringing together artists, writers and poets to produce their works as a joint project in more than 40 books, so Picasso illustrated the works of Max Jacob etc.
In fact as an art and literary publisher he had no equal; being the first to publish works by Max Jacob, Guillaume Apollinaire, André Masson, Gertrude Stein, Pablo Picasso, and many others and in so doing launched many literary careers.
So strong a business ability did Kahnweiler have that by the 1950s his art gallery was in the top 100 companies in France in terms of turnover from export. 
The painting depicts Daniel-Henry Kahnweiler, who owned an art gallery in France.
Gifted by Mrs. Gilbert W. Chapman in memory of Charles B. Goodspeed in 1948 to Art Institute of Chicago. 
Daniel-Henry Kahnweiler (25 June 1884 - 11 January 1979), was a German-born art historian, art collector, and one of the premier French art dealers of the 20th century.
He became prominent as an art gallery owner in Paris beginning in 1907 and he was among the first champions of Pablo Picasso, Georges Braque and Cubism. 
Galerie Louise Leiris was a fine art gallery in Paris established by Daniel-Henry Kahnweiler in 1920.
Initially, the business was known as the Galerie Simon.
It was named after Kahnweiler's partner, André Simon.
In 1940, the business was turned over to Louise Leiris, who was Kahnweiler's sister-in-law.
It was run under her name.
Prominent among the artists who sold paintings through this gallery was Pablo Picasso.
The gallery continued in business for several decades. 
But although the survival and financial success of his artists was important, it was as an art historian and as an eyewitness to Cubism (1907–1914) and to art history to which he made his major contribution.
When working in Paris his spare time was devoted to reading and understanding the history of art and aesthetics.
He also spent his time visiting the city's museums and art galleries; including trips around the continent to see what was being shown in museums and art galleries outside France.
He gave his first interview in 1912 on Cubism, and it was actual historical events that led to his career as a historian.
There is a view that Kahnweiler's sensibility was such that his gallery, and the way he styled and developed it, was as much a cubist gallery as were the paintings by Picasso and the other cubist painters.
The gallery had a clear aesthetic position, uncompromising integrity, financial stability and creative development.
During the years 1907-1914 - the central cradle for Cubism was his gallery - not only to see the works but where one also met the artists, discussed art and where artists discussed each other's works.