Answer by taking a quote from the following article:

Camille Pissarro (French: [kamij pisaRo]; 10 July 1830 - 13 November 1903) was a Danish-French Impressionist and Neo-Impressionist painter born on the island of St Thomas (now in the US Virgin Islands, but then in the Danish West Indies). His importance resides in his contributions to both Impressionism and Post-Impressionism. Pissarro studied from great forerunners, including Gustave Courbet and Jean-Baptiste-Camille Corot. He later studied and worked alongside Georges Seurat and Paul Signac when he took on the Neo-Impressionist style at the age of 54.

When Pissarro returned to his home in France after the war, he discovered that of the 1,500 paintings he had done over 20 years, which he was forced to leave behind when he moved to London, only 40 remained. The rest had been damaged or destroyed by the soldiers, who often used them as floor mats outside in the mud to keep their boots clean. It is assumed that many of those lost were done in the Impressionist style he was then developing, thereby "documenting the birth of Impressionism." Armand Silvestre, a critic, went so far as to call Pissarro "basically the inventor of this [Impressionist] painting"; however, Pissarro's role in the Impressionist movement was "less that of the great man of ideas than that of the good counselor and appeaser ..." "Monet ... could be seen as the guiding force."  He soon reestablished his friendships with the other Impressionist artists of his earlier group, including Cezanne, Monet, Manet, Renoir, and Degas. Pissarro now expressed his opinion to the group that he wanted an alternative to the Salon so their group could display their own unique styles.  To assist in that endeavour, in 1873 he helped establish a separate collective, called the "Societe Anonyme des Artistes, Peintres, Sculpteurs et Graveurs," which included fifteen artists. Pissarro created the group's first charter and became the "pivotal" figure in establishing and holding the group together. One writer noted that with his prematurely grey beard, the forty-three-year-old Pissarro was regarded as a "wise elder and father figure" by the group. Yet he was able to work alongside the other artists on equal terms due to his youthful temperament and creativity. Another writer said of him that "he has unchanging spiritual youth and the look of an ancestor who remained a young man".

Was was the impressionism?
Impressionist style he was then developing,